Familiality in breast cancer: a case-control study in a Sweden population.

1330 consecutively diagnosed breast-cancer patients, and an equal number of paired aged-matched controls without breast cancer, were investigated for a familial history of breast cancer. Patients and controls received identical questionnaires. One relative or more with breast cancer was reported by 18.6% of the patients and by 12.3% of the controls, giving a standardized relative risk (SRR) of 1.6 (P < 0.01). One or more first-degree relatives with breast cancer were reported by 11.2% of the patients and by 6.8% of the controls, with an SRR of 1.7 (P < 0.01). For second-degree relatives the SRR was 1.5 (P < 0.05). Of the patients, 3.9% had mothers with breast cancer compared to 2.7% of the controls (SRR = 1.4, N.S.). One or more sisters with breast canceer were reported by 10.1% of the patients and by 5.1% of the controls (SRR = 2.0, P < 0.01). No distinct difference in familiality between the different age groups was found.