A comparison of chromosomal anomalies detected in 30 cases of breast cancer in females with near-diploid karyotypes is reported. The tumors, of which 20 were previously unpublished, were selected for the relatively low complexity of their karyotypes, among a sample of 118 cases. Almost all of the 151 structural rearrangements detected were unbalanced, and 67% of breakpoints were located in or had contact with heterochromatin. In cases with few anomalies, rearrangements of chromosomes 1 and/or 16 were very frequent, leading principally to a gain of 1q and loss of 16q. In cases with more anomalies (5-16), deletions involving 17p, 4p, 13, 6q, 8p, 9p, 11p, and 11q and gains of 1q and 8q were the most frequent. Homogeneously staining regions (HSR) were detected in 14 tumors, mostly on 8p (6/22) and chromosome 19 (3/22). No double minutes (dmin) were observed. We conclude that trisomy 1q and monosomy 16q are early chromosomal changes in breast cancer, whereas other deletions and gain of 8q are clearly secondary events.