Urine for the analysis of pregnanediol was collected weekly for 3 months from 209 menstruant women aged 11-24 years who lived with their parents and from 59 women aged 17-23 years who had left the parental home. Menstrual cycles were classed as ovulatory if the 24-h pregnanediol output in the 12 days preceding menstruation was greater than or equal to 5 mumol on a single occasion or if the total excreted on 2 days, 1 week apart, was greater than or equal to 7 mumol. In the first group, ovulatory incidence increased with menarchal age. Unfailing ovulation occurred in 22.9, 25.0, 44.8, 42.9, 63.2, 71.8 and 82.6% of those who were less than 1, 1- less than 2, 2- less than 3, 3- less than 4, 4- less than 5, 5-8 and 9-12 years from menarche. Comparable figures for the women who lived in flats and hostels were 40.0% (menarchal age, 5-8 years) and 78.6% (9-12 years). It is concluded that a regular pattern of ovulatory menstrual cycles is established in most young women within 5 years of the menarche, and that departure from the family home is often associated with a regression to a juvenile pattern of anovulatory menstrual cycles.