WHO Collaborative Study of Neoplasia and Steroid Contraceptives. Combined oral contraceptives and gallbladder cancer. International Journal of Epidemiology 1989, 18: 309–314. The relationship between combined oral contraceptives and primary gallbladder cancer was examined in 58 cases and 355 controls participating in an international hospital-based, case-control study. Use of combined oral contraceptives at any time was not associated with the risk of developing gallbladder cancer (RR = 0.6, 95% Cl 0.3, 1.3), and no clear patterns of risks were observed according to the duration of use, or time since first or most recent use. There was also no evidence that oral contraceptives effected risk of gallbladder cancer in women with or without a prior history of gallbladder disease. A history of gallbladder disease or gallstones was a strong risk factor for gallbladder cancer (RR = 2.3, 95% Cl 1.2, 4.4).