Benign breast disease (BBD) is a heterogeneous group of benign breast problems that has been associated with breast cancer risk by several investigators. Genetic alterations have been described in breast carcinomas under the headings of loss of heterozygosity (1p, 3p, 7q, 11p, 17p, 17 and 18q), mutations (p53, c-H-ras-1), and/or gene amplifications (c-myc, int-2/FGF3, and c-erbB-2/neu). In an attempt to determine whether these genetic alterations might also be involved in the development of BBD, we have analyzed such alterations in 50 BBD lesions. The histological types of samples studied were: 37 fibroadenomas; 8 benign phyllode tumors; and 5 fibrocytic diseases. Cellular DNA was extracted from tissues and from corresponding blood leukocytes according to standard techniques, digested with appropriate restriction endonucleases, and analyzed by Southern blot. The following are informative cases found in a total number of patients analyzed for each locus: 13 of 26 for L-myc (1p); 9 of 23 for THRB (3p); 11 of 29 for met (7q); 27 of 50 for c-H-ras-1 (11p); 3 of 13 for TP53 (17p); 14 of 50 for D17S30 (17p); 20 of 33 for D17S4 (17q); and 13 of 33 for D18S5 (18q). No loss of heterozygosity was detected at any of the examined loci. Alternatively, none of the 50 BBD cases displayed an amplification of the three genes tested (c-myc, int-2/FGF3, and c-erbB-2/neu). Our results show that molecular alterations, which are more frequently involved in malignant breast carcinomas, do not occur in BBD lesions. These results indicate that these molecular alterations could constitute late events in the pathogenesis of breast carcinomas.