Anticipation describes an inheritance pattern within a pedigree with an increase in disease severity and/or decrease in age at onset in successive generations. The phenomenon of anticipation has recently been shown to be correlated with the expansion of trinucleotide repeat sequences in a neuromuscular disease, various neurodegenerative disorders and mental retardation. We have studied parent-offspring differences in age at onset and disease severity in 31 pairs with unilineal inheritance of unipolar affective disorder (UPAD). Life-table analyses showed a significant decrease in survival to 1st episode of major depression in the offspring generation compared with the parental generation (P = 0.0007). There was also a significant difference in age at onset (P < 0.001) between parents and offsprings. The offspring generation experienced onset 15.6 years earlier and illness 1.5 x more severe than did the parent generation. Furthermore, there was a significant correlation (P < 0.05) in age at onset between parent and offspring generations. When we excluded pairs where the affected parent has an age of onset greater than the age of the child at the time of ascertainment (i.e., 23 pairs left), there was still a significant (P = 0.02) decrease in age at onset (8.4 years) and 1.5 x more severe disease in the offspring generation. No evidence for specific maternal or paternal inheritance was found. We found evidence of anticipation in 75-80% of this sample of unilineal family pairs of UPAD. Anticipation is, thus, an inheritance pattern in a large group of UPAD which suggests that the expansion of trinucleotide repeat sequences is a possible mode of inheritance in this group of UPAD. The findings of anticipation in this study of families with UPAD and previous findings in families with BPAD suggest that the variable expression of unstable expansions of trinucleotide repeats may turn out to be the basis of the continuum of liability in affective disorders.