Objectives.- The GRECO study has collected data on pregnancies, regardless of their outcome, that occurred in women taking an oral contraceptive. Patients and methods.-The analysis concerned 551 women prospectively recruited in services of gynaecology or obstetrics, termination of pregnancy centres, family planning centres or consultations of gynaecology in France throughout 2002 and who were 12 weeks pregnant or less. Results.-Contraception used during the cycle of conception was an estroprogestative combination in 88% of cases, a microprogestative in 8.7% , a macroprogestative in 0.9% or another type of pill in 2.4% . Progestatives were levonorgestrel 59.0% , gestoden 17.2% , desogestrel 4.7% , norethisterone acetate 2.9% , norgestimate 1.8% , cyproterone acetate 2.0% , norgestrel 1.6% . When asked about the potential cause of the oral contraceptive failure, 76.9% of women reported events such as missed pills which were the most frequent cause of failure (60.8% of failures and 80.1% of events, 2.7 ± 2.7 missed pills), followed by vomiting and diarrhoea. 81.5% of women chose to terminate their pregnancy. Discussion and conclusion.-The GRECO study, despite its limitations (retrospective collection of missed pills data, declaratory data)-showed that missed pills, even once, were the most common reason for oral contraceptive failure. The most frequent decision was the termination of pregnancy. Objectives .- The GRECO study has collected data on pregnancies, regardless of their outcome, that occurred in women taking an oral contraception. Patients and methods. -The analysis concerned 551 women prospectively recruited in services of gynaecology or obstetrics, termination of pregnancy centers, family planning centers or consultations of gynaecology in France during 2002 and who were 12 weeks pregnant or less. Results. - Contraception used during the cycle of conception was an estroprogestative combination in 88% of cases, a microprogestative in 8.7%, a macroprogestative in 0.9 % or another type of pill in 2.4%. Progestatives were levonorgestrel 59.0%, gestoden 17.2%, desogestrel 4.7%, norethisterone acetate 2.9%, norgestimate 1.8%, cyproterone acetate 2.0%, norgestrel 1.6%. When asked about the potential cause of the oral contraceptive failure, 76.9% of women reported events such as missed pills which were the most frequent cause of failure (60.8% of failures and 80.1% of events, 2.7 ± 2.7 missed pills, followed by vomiting and diarrhea. 81.5% of women chose to terminate their pregnancy. Discussion and conclusion .- The GRECO study, despite its limitations (retrospective collection of missed pills data, declaratory data) - showed that missed pills, even once, were the most common reason for oral contraceptive failure. The most frequent decision was the termination of pregnancy.