Register-based studies show that women with ovarian cancer are at increased risk of developing breast cancer. Primary suggested explanations are heredity factors and a common hormonal aetiology. However, clinical surveillance that is provided for cancer patients during, and after, treatment of their primary malignancies together with possible mistakes in the registering procedures could affect the risk estimates. In order to examine these factors in women registered with ovarian cancer who develop subsequent breast cancer, a case-control study was performed. Using a regional Swedish cancer registry including 5060 women registered with ovarian cancer, 89 cases of breast cancer were found. After corrections for discrepancies in the registered and recorded information, 75 cases remained, of which 72 cases were included in the study. Information concerning possible risk factors were extracted from hospital records and compared with 177 matched controls. Suggested risk factors such as parity (relative risk (RR)=1.41), late age at menopause (52-61 years; RR=1.61) and heredity for breast and/or ovarian cancer (RR=1.50) were all connected with a non-significant increased risk of subsequent breast cancer. In all, 43% of the breast cancer cases were revealed without preceding symptoms at clinical follow-up, indicating that increased clinical surveillance is a factor of importance when explaining the increased risk. The fact that only 75 (missing records included) out of the 89 registered breast cancer cases could be linked to the preceding ovarian cancer indicates that the actual risk of developing breast cancer is smaller than previously described. The clinical implications from these findings could be that, beside general screening programmes and health controls offered to women in cancer-prone families, additional mammography examinations based on the assumption of an increased risk of breast cancer are not warranted in ovarian cancer patients.