Twenty-five human breast cancers, surgically resected, were studied by cytogenetic analysis and DNA flow cytometry (FCM). The establishment of karyotypes showed that multiple cell populations probably were derived from a single ancestor clone, because common marker chromosomes always could be demonstrated. Differences of up to 30% were observed when the estimates of DNA content by the two methods were compared. A general tendency toward the acquisition of large marker chromosomes should be at the origin of this discordance, as the proportion of markers for each case correlated significantly with the magnitude of the difference. Parallel use of the two methods revealed the existence of tumors with DNA diploid FCM profiles and highly abnormal hypodiploid karyotypes (35-40 chromosomes), which may explain the limited value of DNA ploidy as an independent prognostic factor in breast cancer.