The aims of this study were to compare genetic aberrations in primary soft-tissue sarcomas and their local recurrences and to evaluate the genetic changes occurring during tumor progression. A primary soft-tissue sarcoma and its subsequent local recurrence were analyzed in 20 tumor pairs by comparative genomic hybridization. The samples were obtained before application of radio- or chemotherapy. Copy number aberrations were detected in 50% of the primary tumors and in 70% of the local recurrences. In primary tumors, the mean number of changes was 2.45 (range, 0 to 11) whereas in local recurrences, it was 5.05 (range, 0 to 17). The mean increase of changes from primary tumor to local recurrence was 2.6 per tumor pair (P =.02). Gains predominated over losses in both primary tumors and their local recurrences. The number of high-level amplifications was twofold in local recurrences. The most frequent gain affected 5p14-p15.1 (10% of primary tumors, 25% of local recurrences) and the most frequent loss, 9p (9p21-pter in 5% of primary tumors; 9p22-pter in 30% of local recurrences). In conclusion, our results show an increase in the number of genetic changes in local recurrences, due to tumor progression. Loss at 9p and gains at 5p and 20q were more frequent in local recurrences, and high-level amplification of 18p11.3 was not detected in any of the primary tumors. Although all these alterations were not specific to local recurrences, they may represent changes important during tumor progression.