Genomic alterations of c-myc (amplification, rearrangements and hypomethylation) were investigated in 30 breast carcinomas and 20 cervix carcinomas. In breast carcinomas c-myc alterations were compared with overexpression of the c-erbB2 protooncogene and the proliferation marker Ki-67. Alterations of c-myc were found in 50% of the breast carcinomas and in 25% of the cervix carcinomas. In 23% of the breast carcinomas c-erbB2 overexpression was associated with c-myc alterations. In 17% of the cases there was overexpression of c-erbB2 without detectable alterations of c-myc. Hence, in 67% of breast cancers alterations of c-myc and/or c-erbB2 have been found, while in 81% of the samples Ki-67 expression was increased. The results suggest that the study of c-myc alterations provides an important complement to that of other prognostic indicators of breast cancer such as c-erbB2 expression.