A correlation analysis was performed on 125 cytogenetically characterized breast cancer cases to assess the relationship between the tumor karyotype and clinicopathologic features. The carcinomas of young women had a higher modal chromosome number than those of older women. The number of chromosomal aberrations and modal chromosome number were also found to correlate with the histologic type, grade and mitotic activity of the tumor. Whereas all lobular carcinomas were karyotypically normal or near-diploid, more than 3 aberrations and sometimes near-triploid or near-tetraploid karyotypes were common findings in ductal carcinomas, especially in grade-III tumors and in tumors showing high mitotic activity in vivo. Karyotypes with cytogenetically unregulated clones and unbalanced structural chromosomal rearrangements were more frequent in infiltrating than in in situ carcinomas but, at least as far as the second of these 2 characteristics is concerned, especially in infiltrating carcinomas that also had an in situ component. The presence of cytogenetic polyclonality correlated with tumor grade. Although recurrent chromosome aberrations were significantly more common in ductal than in lobular carcinomas, none of these breast cancer-associated anomalies seemed to be specific for any particular clinicopathologic parameter. The associations between modal chromosome number and mitotic activity and between cytogenetic polyclonality and tumor grade were found to be statistically significant in multivariate models. No correlations was seen between the karyotypic findings and tumor size or the presence of axillary-lymph-node metastases.