We studied the prevalence and degree of tumor cell infiltration (TCI) in bone marrow (BM) aspirates of 89 infants with stage 4/4 S neuroblastoma and correlated them with MYCN gene status and patient outcome. TCI was scored 0, +, ++, and +++, the last corresponding to an infiltration greater than 10%. TCI 0 was more frequent in stage 4 than in stage 4 S. TCI + and ++ were equally represented. TCI +++ was found only in 9 patients, all with typical stage 4 features (bone or lung involvement). Overall survival was not significantly influenced by the presence and degree of TCI.