Age at menarche shows a downward secular trend and differs according to socioeconomic conditions, presumably the result of dietary variations. We report two studies conducted in Quebec City in which the relation between diet and menarche was evaluated. In 1978-1980, baseline information for these studies was obtained on girls aged 9 through 15 and included body weight, height, fatfold thickness at six sites, percent of body fat (derived from underwater weighing), and a 3-day dietary record. In a cross-sectional study of 207 girls aged 11 through 15, we compared dietary patterns between premenarcheal and menarcheal girls using logistic regression. In a 6-year follow-up study of 109 girls who had not experienced menarche at the baseline data collection, the relation between time of occurrence of menarche and dietary patterns was assessed using proportional hazards models. The results confirm the association between body weight and age at menarche. Body fat, however, was not strongly related to the onset of menstruation. In both studies, a higher dietary energy intake was associated with an earlier age at menarche. Dietary composition, and dietary fat in particular, was not related to menarche.