diff --git a/tests/integration/test_cross_entropy_loss.py b/tests/integration/test_cross_entropy_loss.py
new file mode 100644
index 000000000..040070031
--- /dev/null
+++ b/tests/integration/test_cross_entropy_loss.py
@@ -0,0 +1,32 @@
+import torch
+
+from transformer_lens.HookedTransformer import HookedTransformer
+
+
+def test_cross_entropy_attention_mask():
+    """Check that adding a bunch of masked tokens to the input does not change the loss."""
+    MODEL = "solu-1l"
+    model = HookedTransformer.from_pretrained(MODEL)
+
+    # Step 1: Get the default loss on a prompt
+    prompt = ["The quick brown fox jumps over the lazy dog."]
+    default_tokens = model.to_tokens(prompt)
+    default_attention_mask = torch.ones_like(default_tokens)
+    default_loss = model(default_tokens, return_type="loss")
+    ones_mask_loss = model(
+        default_tokens, attention_mask=default_attention_mask, return_type="loss"
+    )
+    assert torch.allclose(default_loss, ones_mask_loss, atol=1e-6)
+
+    # Step 2: Get the loss when we add some extra tokens to the input and set their attention mask
+    # to zero
+    extra_prompt = ["Lorem ipsum dolor sit amet, consectetur adipiscing elit."]
+    extra_tokens = model.to_tokens(extra_prompt)
+    extra_zeros_attention_mask = torch.zeros_like(extra_tokens)
+
+    combined_tokens = torch.cat([default_tokens, extra_tokens], dim=1)
+    combined_attention_mask = torch.cat([default_attention_mask, extra_zeros_attention_mask], dim=1)
+    combined_masked_loss = model(
+        combined_tokens, attention_mask=combined_attention_mask, return_type="loss"
+    )
+    assert torch.allclose(default_loss, combined_masked_loss)
diff --git a/transformer_lens/HookedTransformer.py b/transformer_lens/HookedTransformer.py
index 3a35fa307..8ee2e74f8 100644
--- a/transformer_lens/HookedTransformer.py
+++ b/transformer_lens/HookedTransformer.py
@@ -587,7 +587,7 @@ def forward(
                     assert (
                         tokens is not None
                     ), "tokens must be passed in if return_type is 'loss' or 'both'"
-                    loss = self.loss_fn(logits, tokens, per_token=loss_per_token)
+                    loss = self.loss_fn(logits, tokens, attention_mask, per_token=loss_per_token)
                     if return_type == "loss":
                         return loss
                     elif return_type == "both":
@@ -600,6 +600,7 @@ def loss_fn(
         self,
         logits: Float[torch.Tensor, "batch pos d_vocab"],
         tokens: Int[torch.Tensor, "batch pos"],
+        attention_mask: Optional[Int[torch.Tensor, "batch pos"]] = None,
         per_token: bool = False,
     ):
         """Wrapper around `utils.lm_cross_entropy_loss`.
@@ -608,7 +609,7 @@ def loss_fn(
         """
         if tokens.device != logits.device:
             tokens = tokens.to(logits.device)
-        return utils.lm_cross_entropy_loss(logits, tokens, per_token)
+        return utils.lm_cross_entropy_loss(logits, tokens, attention_mask, per_token)
 
     @overload
     def run_with_cache(
diff --git a/transformer_lens/utils.py b/transformer_lens/utils.py
index f5d3bacbe..7e5828e14 100644
--- a/transformer_lens/utils.py
+++ b/transformer_lens/utils.py
@@ -115,6 +115,7 @@ def to_numpy(tensor):
 def lm_cross_entropy_loss(
     logits: Float[torch.Tensor, "batch pos d_vocab"],
     tokens: Int[torch.Tensor, "batch pos"],
+    attention_mask: Optional[Int[torch.Tensor, "batch pos"]] = None,
     per_token: bool = False,
 ) -> Union[Float[torch.Tensor, ""], Float[torch.Tensor, "batch pos"]]:
     """Cross entropy loss for the language model, gives the loss for predicting the NEXT token.
@@ -122,6 +123,8 @@ def lm_cross_entropy_loss(
     Args:
         logits (torch.Tensor): Logits. Shape [batch, pos, d_vocab]
         tokens (torch.Tensor[int64]): Input tokens. Shape [batch, pos]
+        attention_mask (torch.Tensor[int64], optional): Attention mask. Shape [batch, pos]. Used to
+            mask out padding tokens. Defaults to None.
         per_token (bool, optional): Whether to return the log probs predicted for the correct token, or the loss (ie mean of the predicted log probs). Note that the returned array has shape [batch, seq-1] as we cannot predict the first token (alternately, we ignore the final logit). Defaults to False.
     """
     log_probs = F.log_softmax(logits, dim=-1)
@@ -129,10 +132,20 @@ def lm_cross_entropy_loss(
     # Offsets needed because we're predicting the NEXT token (this means the final logit is meaningless)
     # None and [..., 0] needed because the tensor used in gather must have the same rank.
     predicted_log_probs = log_probs[..., :-1, :].gather(dim=-1, index=tokens[..., 1:, None])[..., 0]
+
+    if attention_mask is not None:
+        # Ignore token positions which are masked out or where the next token is masked out
+        # (generally padding tokens)
+        next_token_mask = torch.logical_and(attention_mask[:, :-1], attention_mask[:, 1:])
+        predicted_log_probs *= next_token_mask
+        n_tokens = next_token_mask.sum().item()
+    else:
+        n_tokens = predicted_log_probs.numel()
+
     if per_token:
         return -predicted_log_probs
     else:
-        return -predicted_log_probs.mean()
+        return -predicted_log_probs.sum() / n_tokens
 
 
 def lm_accuracy(