Background: Machado-Joseph disease (MJD SCA3), a spinocerebellar ataxia related to expansion of a CAG tract, has already been related to anticipation and meiotic drift. However, fitness of MJD carriers has been little studied. Objective: To analyze genetic fitness of MJD patients, comparing them to their unaffected relatives and to the general population (GP) of origin. Subjects and methods: 182 informants, belonging to 82 MJD families, agreed to participate in the study. Informants supplied data about 828 MJD patients. Number of children (NC), gender, age, school attainment, menarche and menopause were compared between general and emeritus (older than 45 years of age or deceased) groups. Results: Mean NC of the GP and of MJD patients were respectively 1.90 and 2.93± 2.3 (p = 0.0037). Comparisons within families also showed differences: the mean NC of unaffected and affected emeritus MJD women were, respectively, 2.68 and 3.89 (p = 0.0037). Affected MJD women had earlier mean ages at the delivery of their first child and menopause (p