The relationship between ovarian hormones and bleeding patterns during continuous progestin contraception was studied in 29 women who used Nestorone (NES) releasing implants. Oestradiol and progesterone were measured in blood samples taken twice a week for 6 consecutive weeks, during months 6, 12, 18 and 24 of implant use. Retrospectively, the association between hormonal concentrations and bleeding patterns was evaluated. Twenty-four short (</= 24 days), 36 normal (25-35 days), 27 long (36-90 days) and nine episodes of amenorrhoea, as well as 28 periods of prolonged bleeding, were identified. Short cycles, periods of prolonged bleeding and amenorrhoea were characterized by low oestradiol concentrations. Bleeding after a normal or long cycle regularly occurred after a rise and fall of oestradiol or oestradiol and progesterone. The duration of bleeding was directly and significantly associated with the highest concentration of oestradiol in the previous 15 days in anovulatory cycles. Luteal activity was associated with shorter duration of bleeding. Periods of prolonged bleeding were usually initiated with a drop in oestradiol and over half ended with an increase in oestradiol. It was concluded that changes in endogenous hormone have a significant influence over the bleeding pattern during use of NES implants.