Clonal chromosome aberrations identified after short-term culture are presented for 13 chondrosarcomas; in 5 cases both the primary tumors and local recurrences were studied. The stemline chromosome number was hypodiploid or hyperhaploid in 9 tumors. The most frequent numerical anomalies were, in falling order of frequency, loss of chromosomes Y, 10, 13, and 6, and gain of chromosomes 7 and 20. No recurrent structural rearrangement was found, but chromosome bands 5q13, 1q21, 7p11, and 20q11 were each involved in three different rearrangements. Karyotypic heterogeneity was assessed in two different ways: as the presence of more than one clone in one sample and as the presence of different clones in different samples from the same surgical specimen. Clonal karyotypic evolution was demonstrated in 6 of the 7 cases in which two or more samples could be investigated. All 6 showed intersample heterogeneity. Intrasample heterogeneity was found in only 5 of the 28 samples with aberrations. By comparing the incidences of the nonrandomly occurring aberrations in stemlines and sidelines in the heterogeneous tumors, it was possible to conclude that loss of chromosome 13 and rearrangement of band 5q13 were early events in the clonal evolution.