The spectrum of lipomatous tumors differs in the adult and pediatric populations, with liposarcoma being rare in children. Nearly 10% of individuals with Li-Fraumeni syndrome develop sarcomas in the first 2 decades of life; however, the frequency of sarcoma types and subtypes in this syndrome is unknown. Two atypical lipomatous tumors/well-differentiated liposarcomas (ALT/WDLS) were identified in the pathology files of our institution in young children from "classical" Li-Fraumeni and Li-Fraumeni variant kindreds with a known germline TP53 mutation (Y220C) in one of the families. The patients were 5 and 6 years of age and the ALT/WDLSs were the first expression of the syndrome. The tumors had a high degree of cellular atypia and differed from sporadic ALT/WDLS by strong nuclear immunoreactivity for p53 and absent mdm2 expression. This is the first report of 2 ALT/WDLSs presenting in children before 10 years of age, both in association with Li-Fraumeni syndrome/variant. ALT/WDLS in a young child should raise the possibility of a cancer predisposition syndrome and, in this setting, the p53(+)/mdm2(-) immunophenotype might be characteristic. Recognition of this lesion and its association is important for early diagnosis and subsequent tumor surveillance in the proband and affected family members.