Urine was collected at weekly intervals for 3 months from 254 menstruant women aged 15–39 years, all of whom were 4 or more years from the menarche. Menstrual cycles were classed as ovulatory if the 24-hr pregnanediol output was 5 μmol or over on a single occasion, or if the total excreted on 2 days, 1 week apart, was 7 μmol or over. Of the 108 women aged 20–24 years, only 62% ovulated in every cycle compared with 88% for the 58 women aged 25–29 years, and 91% for the 44 women aged 30 or over. Unfailing ovulation occurred more often in non-students (99 women, of whom 83·5% ovulated in every cycle) than in age-matched students (111, 59·5%), and more often in women who lived with relatives (127, 83·5%) than in those living in flats and hostels (72, 51·4%).