OBJECTIVE:To investigate the efficacy, cycle control and tolerability of a phasic oral contraceptive containing ethinylestradiol 35/30/30 microg and desogestrel 50/100/150 microg. METHODS:A multicenter study was conducted involving 2070 healthy, fertile women, who received study treatment for six treatment cycles. RESULTS:Most of the participants (79%) had previously been using an alternative oral contraceptive. In 10,408 treatment cycles, two women became pregnant while on treatment (Pearl index, 0.25). The incidence of irregular bleeding was 10% before treatment, rising to 27% at cycle 1, and decreasing to 11% by cycle 6. Irregular bleeding was mainly due to spotting rather than breakthrough bleeding and the incidence of breakthrough bleeding remained below 2.2% for most of the study period. Only 1.8% of women withdrew due to bleeding irregularities. First-time oral contraceptive users initially experienced more irregular bleeding than switchers but these differences lessened over time. The most common adverse events during treatment were headache, breast tenderness and nausea. The incidence of these adverse events fell to below pretreatment levels with continued use. CONCLUSION:The phasic preparation was effective and well tolerated.