Between 1979 and 1985 surgical resections from 680 cases of primary breast cancer were examined histologically. The patients were divided into four groups: (i) patients aged between 45 and 69 years who had been screened (n = 316); (ii) those younger than 45 who had not been screened (n = 55); (iii) those aged between 45 and 69 who had not been screened (n = 104); and (iv) those older than 69 who had not been screened (n = 205). The material was compared in terms of the association between in situ and invasive carcinoma. There was a low incidence of lobular carcinoma in situ (LCIS) in all groups and a high incidence of ductal carcinoma in situ (DCIS) which declined with age. Ninety nine group i patients had in situ carcinoma or early invasive carcinoma (less than 1 cm in diameter) compared with 19 of group iii cases. Screened patients had fewer multicentric cancers and a lower incidence of large invasive cancer compared with unscreened patients. Group ii patients had a higher incidence of whole quadrant in situ carcinoma, multiple cancer, and lymph node metastases. Group iv patients had a lower incidence of in situ cancer, and more low grade cancer than the other groups. Cases were divided into four types on the basis of this analysis.