Repetition is a pervasive feature of children's environments, and may be an important contributor to learning such complex sequential structures as language. Endress, Dehaene-Lambertz, and Mehler (2007) found that repeated tone sequences were learned more easily than sequences containing ordinal relations, but there have been no direct comparisons of repeating sequences versus sequences that contain similar, but not identical, stimuli. In Experiment 1, we compared learning from repeating tone sequences to learning from tones that varied in similarity, and confirmed that repetition is a special case for learning. In Experiment 2 we showed that the learning distinction between repeated and similar elements is not affected by whether similarity is variable. We conclude by indicating that repetition provides an important constraint on learning, and we discuss the extent to which such constraints are consistent with general-purpose statistical learning mechanisms.