The case of a 7-year-old boy presenting at diagnosis with a localized (stage III) Wilms' tumor of favorable histology is presented. Immunocytologic analysis of bone marrow aspirates revealed cells positive for neural cell adhesion molecule (NCAM) and negative for class I major histocompatibility complex (MHC) antigens. These cells were interpreted as deriving from the tumor blastemal component. Postoperatively the child underwent radiotherapy and chemotherapy, and he remains free of disease 12 months after completion of therapy. In patients with nonmetastatic Wilms' tumor at onset, the evaluation of the actual frequency of occult marrow involvement and the assessment of its clinical significance may necessitate further investigation.