Stolley, P. D. (Johns Hopkins U. School of Hygiene and Public Health, 61 5N. Wolfe St., Baltimore. MD 21205). J. A. Tonascia. M. S. Tockman, P. E. Sartwell, A. H. Rutledge and M. P. Jacobs. Thrombosis with low-estrogen oral contraceptives. Am J Epidemiol 102:197–208. 1975.A retrospective study of the relationship of oral contraception to thrombosis was made in 104 idiopathic cases, 357 other thrombotic cases (exclusive of cerebrovascular thrombosis), and 1302 matched controls. The relative risk for idiopathic case users was 7.2 times that for non-users; for the whole series it was 1.9. The apparent low risk for non-idiopathic cases is considered to be due to selective prescription of this method of contraception. The risk forwomen using formulations containing 100 μg or more of estrogen was higher than that for users of lower doses. Among the controls, the risk of gallbladder surgery was twiceas high for users as non-users. Several theoretical sources of bias were searched for andshown not to be present. Histories of oral contraceptive use were found to be accurate. Adefinite risk of thrombosis exists even with the low-estrogen dosage currently employed inoral contraceptive steroids.