Cytogenetic analysis was performed on a selected series of short-term cultures from 34 patients with documented metastatic breast carcinoma. The majority of tumor cells were hyperdiploid, with clonal structural alterations observed in 94% of patients (32/34). The most common numeric changes were -2, -15, and -18. Chromosome 20 was the most frequently over-represented (in near-3n tumors only). Clonal structural chromosome alterations included isochromosomes, terminal deletions, and, most frequently, unbalanced non-reciprocal translocations. Chromosomes most often involved in structural rearrangements included 1, 7, 11, and 6 (accounting for 24.7%, 10.3%, 9.1%, and 7.0% of breakpoints, respectively). When the breakpoints of clonal structural abnormalities were analyzed, they were shown to cluster to several chromosome segments, including 1p11-q21, 7pter, 11p12-q12, and 6q11-21. An analysis of the net gain or loss of specific chromosome segments was also performed, with the most consistent tendency observed being the over-representation of 1q, 6p, 7, and 11. The most frequent losses included 1p, 6q, 7, and 11q.