The pattern of clonal karyotypic evolution in breast carcinomas carrying an i(1q) or a der(1;16)(q10;p10) as the primary chromosome abnormality was assessed in a series of 42 tumors, including 8 described here for the first time, with either or both (3 tumors) of them defining cytogenetic features. Evidence of clonal evolution was seen in somewhat more than half of all cases in both subgroups. The secondarily acquired aberrations appeared to be nonrandom in distribution. This was especially so for structural rearrangements of 11q leading to loss of material from this arm, which were clearly more common in both subgroups than in karyotypically abnormal breast carcinomas in general. Other deviations from random were less certain but seemed to include the frequent occurrence of +20 in tumors with i(1q) and +7 in tumors with der(1;16)(q10;p10). That differences were observed between i(1q) carcinomas and der(1;16)(q10;p10) carcinomas with regard to their patterns of clonal evolution hints that the pathogenetic effect of the primary change in these two situations may be more than the mere gain of an extra copy of 1q.