To determine whether prior oral contraceptive (OC) use is a risk factor for pituitary prolactinoma, we attempted to identify all women (n = 72) with a prolactinoma diagnosed between 1976 and 1980 in three counties in western Washington. A control group of 303 women was selected by dialing random telephone numbers from the same counties. Prior OC use, according to OC indication, was ascertained during a standardized telephone interview. Relative to the risk for women who had never used an OC, the risk of prolactinoma for women who had used OCs for birth control was 1.3 (95% confidence interval, 0.7 to 2.6). This risk was 7.7 for women who used OCs for menstrual regulation (95% confidence interval, 3.7 to 17.0). Previous findings of an association between OC use and prolactinoma may have resulted from OC treatment of menstrual irregularity in women with an undiagnosed prolactinoma.