College students whose menarche had occurred 7 years previously, prospectively recorded menstrual and sexual behavior history for 14 weeks as well as basal body temperatures. Regular weekly coital activity associates with the highest incidence of fertile type cycles in this sample of young women as follows: (1) Regular weekly heterosexual behavior was associated with 29.5 +/- 3 day menstrual cycle length. Less frequent sexual activity (sporadic) and celibacy were associated with an increased frequency of aberrantly short and long cycle lengths. (2) Either of two heterosexual behaviors (coitus and/or genital stimulation by a man) were behaviors which were adequate as associates of 29.5 +/- 3 day cycles. (3) Self-stimulation (masturbation to orgasm) was inadequate as an associate of the above-described pattern of menstrual cyclicity. (4) Women with regular weekly coital activity had the highest incidence (90%) of fertile type basal body temperature (BBT) rhythms. Sporadically active women had the next highest incidence (55%) of fertile type BBT rhythms. Celibate women had the lowest incidence (44%) of fertile type BBT's.