This study determines the ovarian effects, contraceptive efficacy, and effects on serum levels of norethindrone acetate (NET-Ac) and ethinyl estradiol (EE) among women using a single contraceptive vaginal ring (CVR) cyclically over a period of 1 year. A total of 60 women were enrolled and used the ring according to a schedule of 3 weeks "in" and 1 week "out." Assays of serum norethindrone acetate (NET-Ac) ethinyl estradiol (EE) levels were taken twice weekly in cycles 6, 9, and 13. Despite luteal activity in some cycles, no pregnancies were noted within the 12-month study period. Heavier body weight was associated with increased probability of luteal activity. Mean serum levels decreased over the last 3 months of CVR use, accounting for the increase in luteal activity and possible ovulations in cycle 13. Among women in Sydney, by contrast with women in the other centers, a difference in the effect on lipids was seen. However, the changes in lipid levels were very small. The side effects were a little different from those experienced by women using a combined pill. Nausea and vomiting were largely confined to early cycles and most common in the first days of the first cycle. Weight gain was also not a problem, although there was a small mean increase in body weight over the 12-month treatment period. This study indicates that use of a single CVR releasing EE and NET-Ac over a period of 12 months constitutes an acceptable, safe and effective contraceptive method.