To determine the effectiveness of contraceptive use a two year audit of pregnant women registered in one group practice was carried out. The methods of contraception used by women with unplanned pregnancies were studied and the rates of failure assessed. Of the 518 pregnancies during the study, 187 (36%) were unplanned. Unplanned pregnancies were most common in the 15-19 age group (54 out of 187), and women aged under 25 used contraceptives less reliably than women aged 25 and over. The combined pill was the most effective method of contraception in all age groups. The methods that resulted in most unplanned pregnancies were the sheath in women aged 25 and over and incorrect use of oral contraceptive or no contraception in those aged under 25. The fear of side effects was an important reason why women did not use the combined pill, being cited by 22 out of 134 women, and inappropriate medical advice was cited by a further 20 women. More discussion between doctors and patients and readily available information on the use of oral contraceptives might help to reduce the number of unplanned pregnancies.