One hundred and two bone marrow samples were analysed by histological and immunohistochemical methods for neurone specific enolase (NSE). The biopsies were performed to determine the extent of bone marrow disease in 84 neuroblastomas, nine embryonal rhabdomyosarcomas, five Ewing's sarcomas, two cases of Hodgkin's disease and two lymphoblastic lymphomas. Twenty seven (32%) of neuroblastoma bone marrows showed metastases by conventional histological techniques and 33 (39%) after immunohistochemical staining with NSE. Five embryonal rhabdomyosarcomas, five Ewing's sarcomas, and two lymphoblastic lymphomas showed bone marrow metastases. Only one of these cases was reactive for NSE. NSE represents a very sensitive immunomarker for the follow up of neuroblastoma and improves detection of bone marrow invasion by neuroblastoma.