The Active Drug Monitoring project aims to assess the positive and negative effects of drugs following commercialisation. It was decided to use a study on "series of cases" in women who were taking an oral contraceptive containing 0.075 mg of Gestodene and 0.030 mg of Ethynylestradiol (Ginoden, Schering; Minulet, Wyeth). Twenty-seven centres took part in the study. 1166 women were recruited with a total of 10320 cycles studied. A data base was set up on a PC which was then used by each participating centre. Data were transmitted from the various centres using floppy disks. On enrollment a detailed medical history was recorded and, in addition to a careful objective examination, a blood sample was taken to assess biochemical parameters. The monitoring protocol consisted in the assessment of biochemical parameters at the start and end of treatment (12 months), as well as blood pressure, weight and menstrual characteristics after 3, 6 and 12 months. At each visit patients were questioned about the possible onset of collateral effects and their intensity. At the end of the study a reduction was noted in the mean cycle length (29.34 vs 27.45 days) and a parallel reduction in the quantity of menstruation. In spite of an increase in all the biochemical parameters examined, only HDL-cholesterol was statistically significant (p less than 0.01). 122 women decided not to complete the study, but only 72 due to side effects. It is however interesting to note that this decision was usually not related to the severity of symptoms.