The first menstrual cycle after a therapeutic abortion was studied in eighteen women by measuring plasma oestradiol and progesterone concentrations and the urinary excretion of LH until the first menstruation. The plasma concentrations of oestradiol and progesterone declined rapidly, reaching low values as early as the second day after the abortion. An increase in plasma oestradiol levels was observed from the seventh post-abortal day onwards. P peak of short duration in the plasma progesterone concentrations occurred during the increased oestradiol synthesis. The concentration of progesterone during the luteal phase of the cycle was lower than in a normal cycle, indicating an inadequate luteal function. Return of ovulation was observed in 83% of patients during the first cycle; as judged by a biphasic oestradiol profile in plasma, a mid-cycle LH peak, and subsequent elevation of progesterone. The first LH peak was observed 16 days after the abortion. The rapid return of the ovulatory function makes immediate post-abortal contraception a necessity.