The possible selection bias due to the use of different series of controls in case-control studies has been analysed in the Norwegian part of a joint national case-control study in Sweden and Norway of oral contraceptive (OC) use and breast cancer in young women. A total of 105 cases diagnosed 1984-1985 were interviewed in addition to 210 population controls matched for age, 104 neighbourhood controls matched for age and postal address, and 96 hospitals controls matched for age. The relative risk was for each of the sets of controls 1.0 for 8 years and more of total use of OCs (adjusted for age at menarche, parity, age at first full term pregnancy, history of breast cancer in mother, history of benign breast biopsy). An additional questionnaire was sent to those who did not wish to participate (non-attenders). A slight selection bias was found for the population controls with more nulliparous never-users of OCs among the non-attenders than among interviewed controls.