Menstrual patterns were studied longitudinally in 140 adolescent Swiss girls. The mean cycle length was 32.2 (S.D. 11.24) d in the first year after menarche, and 29.9 (S.D. 7.12) d in the sixth year after menarche. The values of the mean cycle length became essentially constant from the sixth post-menarcheal year, the chronological age of 20 y and the 36th post-menarcheal cycle. No association was found between cycle length and age at menarche. The menarcheal age of mothers and that of their daughters were significantly correlated with each other (r = 0.29, P less than 0.001). The most frequent cycles of 21-27 and 28-34 days constituted 64-81% of all cycles depending on post-menarcheal age. Short cycles (14-20 d), representing 2-6% of all cycles, were present in 14-31% of the girls; long cycles (35-41 d), constituting 10-16% of all cycles, occurred in 46-68% of the girls. The frequency of the long cycles tended to decrease at higher post-menarcheal ages. There were minimal changes in the duration of menstrual flow throughout the first six years after menarche. The mean duration was 5.4 (S.D. 1.72) d for the first year, and 5.1 (S.D. 1.33) d for the sixth year. The most frequent flow duration was 3-7 d, present in 88-94% of cycles. Prolonged bleeding tended to decrease with rising post-menarcheal and chronological age. The intensity of menstrual flow was mild in 11-16%, moderate in 62-78% and severe in 11-25% of the girls. The frequency of these three categories remained essentially unchanged during the first five post-menarcheal years.