Basal temperature records were maintained by 346 women after ceasing to take oral contraceptives; 167 had ceased using combined and 179 had ceased using low-dose sequential formulations. In the first cycle after withdrawal of low-dose sequential pills, 11 (6%) women failed to ovulate and ovulation was delayed in five cases. Short luteal cycles occurred in 14 subjects. In 83% of the subjects there was no disturbance of ovulation. In the first cycle after withdrawal of combined oral contraceptives, 49 (30%) women failed to ovulate and ovulation was delayed in 29 cases. A short luteal phase was observed in 26 subjects. In only 38% of the subjects was there no disturbance of ovulation. The minimal disturbance to ovulation in the cycle after the withdrawal of low-dose sequential formulation indicates that this type of oral contraceptive carries little risk of producing post-pill infertility. Women under the age of 21 years were anovulatory nearly twice as frequently as older women after ceasing to use combined oral contraceptives.