Oral contraceptive agents (OCs) containing levonorgestrel, 150 micrograms, and ethinyl estradiol, 30 micrograms, account for almost half the sales of such agents in the United Kingdom. The remarkable success of this formulation has occurred primarily because it provides extremely low doses of both hormonal constituents yet still gives most users a very acceptable bleeding pattern: the incidence of breakthrough bleeding is about 6% and of amenorrhea, less than 3%. Approximately 90% of users have cycle lengths of 28 +/- 3 days. The risk of serious side effects is significantly lower than with formulas containing higher doses of progestogen and/or estrogen. Minor side effects occur in only a small percentage of cycles. Headache is reported in approximately 10% of all cycles and should be regarded as a potential indicator of increased risk. If it presents as focal migraine, use of the combined OC should be discontinued. The contraceptive effectiveness of the 150/30 formulation is similar to that of the 50 micrograms formulations among compliant women; in less compliant women the margin of error is reduced, and the possibility of an increased risk of accidental pregnancy must be considered.