Results of a previous case-control study in Slovenia showed a significantly elevated risk of breast cancer for ever-OC users aged 25 to 54 years. A further study was conducted in 1988-1990 in the whole of Slovenia, employing more rigorous epidemiological methodology. Cases were 624 women with breast cancer, aged 25 to 54 years, diagnosed at the Institute of Oncology in Ljubljana and other Slovenian hospitals. Controls were 624 women identified through the Population Registry, randomly selected and matched with cases by date of birth and commune of residence. Data were collected by personal interview, using coloured photographs of packages of all OC on the Slovenian market since 1964. A calendar of reproductive life events was constructed with participants to improve estimation of exposure. The adjusted odds ratio (OR) for ever-users was 1.09. There was no increase in risk with total duration of use, interval since first use, age at starting OC, according to use before or after first delivery and time between menarche and age at first use. Increased risk (OR = 2.92) was found for OC users at the time of diagnosis and for those stopping them less than 6 months before (current users). The risk was not increased for those who stopped OC more than 6 months before diagnosis. The results of this study are consistent with most studies showing no overall effect of OC in women aged till 55 years ever using them. Increased risk of breast cancer in current OC users suggests a possible promoting effect of the pill in susceptible women, and indicates the need for careful breast surveillance of these women while they are using OC and in the period immediately following cessation.