In an artificial grammar learning task, amnesic patients classified test items as well as normal subjects did. Item similarity did not affect grammaticality judgments when similar and nonsimilar test items were balanced for the frequency with which bigrams and trigrams (chunks) that appeared in the training set also appeared in the test items. Amnesic patients performed like normal subjects. The results suggest that concrete information about letter chunks can influence gramaticality judgments and that this information is acquired implicitly. The similarity of whole test items to training items does not appear to affect grammaticality judgments.