Over four years the histological features of benign breast diseases, diagnosed after biopsy of non-palpable mammographic abnormalities, were reviewed and correlated with the mammographic appearances. The histological features were compared with those from all other benign biopsy specimens taken during the same period. The incidence of sclerosing adenosis and microcalcifications was considerably higher in the group of non-palpable mammographic lesions; fibrous disease of the breast and radial scar (infiltrating epitheliosis) were also more common. There was no difference in the incidence of epithelial hyperplasia between the two groups. Correlation with the mammographic appearances showed that microcalcification was most often associated with blunt duct adenosis and that stromal distortion or masses were most often caused by fibrous disease.