Recently, a new form of human mutation-expansion of trinucleotide repeats-has been found to cause fragile X syndrome, Huntington's disease and other neurodegenerative diseases. These diseases are characterized by unusual patterns of inheritance, in particular, genetic anticipation in which the severity of the disorder increases and the age at onset decreases in successive generations of a pedigree. This phenomenon, formerly ascribed to observation biases, correlates with the expansion of trinucleotide repeat sequences. Two recent studies indicate that anticipation is present in familial schizophrenia. These findings support both an active search for unstable trinucleotide repeat sequences in schizophrenia and reconsideration of the genetic models used in this disorder.