Eleven ductal carcinoma in situ (DCIS), 5 DCIS with microinvasion and 4 benign lesions have been investigated cytogenetically. Twelve of the 20 breast tumors (60%) had sufficient mitotic activity for chromosome analysis. All ductal carcinoma in situ had abnormal karyotypes, and clonal marker chromosomes could be identified in all tumors analyzed. None were cytogenetically normal. All but one of the 12 DCIS showed genetic heterogeneity. Tumor progression seems to be associated with loss of chromosomes.