The factors associated with unplanned pregnancies among oral contraceptive (OC) users were investigated in the 8058 women admitted to University Hospital (Copenhagen, Denmark) during 1986-91 for induced abortion. 70 (0.87%) of these women had been consistent OC users; women with a history of missing 1 or more days of pill use and users of gestagen-only pills were excluded. The largest subgroup (41.4%) of these women used triphasic OCs; 35.7% used a low-dose OC and the remainder took high-dose, two-phased, or unknown preparations. Concomitant use of one or more other drugs (spray saline, astemizol, mianserin, chlorcyclizin, paradryl, carbamazepin, lithium, chlorprotixen, and imipramine was reported by 5 women (7%) who experienced OC failure; 4 of these women were using a triphasic OC. At the time of conception, 16 women (30%) had symptoms of gastroenteritis (vomiting and/or diarrhea) and had self-medicated with an unknown preparation. No risk factor could be identified in the remaining 49 women (70%). 2/3 of women who became pregnant while taking OCs returned to this method after abortion. There was no association between the annual sales of various OCs during the study period and the incidence of OC failure. These findings confirm the contributions of concurrent gastroenteritis and drug interactions to OC failure but suggest these are rare occurrences.