A search of the centralized data resource available at the Mayo Clinic for all cases of pituitary adenoma diagnosed in the population of Olmstead County, Minnesota, disclosed an increasing incidence of this tumor in women of childbearing age. The sex, age, and temporal relationships suggest that, if this increase is real, oral contraceptives should be considered as one of the possible etiologic factors. A case-control study, however, did not reveal an association of prior use of oral contraceptives with pituitary tumor--relative risk, 0.5; 95% confidence interval, 0.1 to 2.2. No association was found with other possible risk factors, i.e., prior head injury, radiation therapy, seizures, and smoking. Thus, unless other etiologic agents can be identified, it appears that the increasing incidence is due to advances in diagnostic and surgical technology rather than to a specific etiologic factor.