A complex segregation analysis was conducted on two British series (one consecutive series of probands with breast cancer and one series ascertained through a normal consultand). Altogether there were 1248 nuclear families with breast cancer. A dominant gene with a frequency of 0-003 giving a lifetime penetrance of 0-83 is favoured. Ovarian, endometrial and cancers associated with the SBLA syndrome, as well as benign breast disease, were significantly more common in familial breast cancer than in families of single cases. Probands in families with more than one individual with breast cancer were non-significantly younger than isolated probands.