Fibrosarcomas occurring in infants and young children are intriguing tumors. Although biologically more benign than their counterparts occurring in old patients, they are histologically similar. Microscopically, they fit in one end of a spectrum of fibrous proliferations occurring in the pediatric population. Prompted by recent karyotypic reports of nonrandom gains of chromosomes 8, 11, 17, and 20 in infantile fibrosarcomas, we retrospectively analyzed 12 infantile fibrosarcomas utilizing fluorescence in-situ hybridization (FISH) with chromosome 8-, 11-, 17-, and 20-specific probes. Parallel studies were performed on fibrosarcomas occurring in older children and young adults and also on cellular fibromatoses, myofibroma-toses, and fibromatoses. Gains of chromosomes 8, 11, 17, and 20 (in various combinations) were observed in 11 of 12 fibrosarcomas occurring in infants <2 years of age. Extra copies of chromosomes 17 and 20 were observed in a fibrosarcoma occurring in a 5-year-old child but no abnormalities were detected by FISH in four additional fibrosarcomas occurring in patients aged 6–17 years. One of three cellular fibromatoses was characterized by extra copies of chromosome 8, 11, 17, and 20. Similar findings were not observed in any of the noncellular fibromatoses or in myofibromatoses.