On 80 occasions 4 iliac biopsy trephines and 4 iliac aspirations were performed in 37 children with neuroblastoma at various stages of the disease. In 38 of these procedures, tumour cells were detected. In 24% of cases, both trephines and aspirates were positive, whereas in 63% neuroblastoma cells were detected only on the trephines and in 13% only on the aspirates. In addition, in 37% of the stagings, only one out of the 8 investigations was abnormal. Only in one of 33 pathological cases, was BM involvement diagnosed on trephine imprint. No involvement was ever observed on tibial and sternal aspirates without iliac involvement. Immunological studies with two monoclonal antibodies HSAN 1-2 and UJ13A were performed on 56 occasions. Cytohistological and immunological studies were concordant in 39. In 3 studies, the antigens recognized by the two monoclonal antibodies were not expressed by the initial tumour and in 3 additional studies immunological results were falsely negative; but in 11 cases monoclonal antibodies identified residual malignancy despite normal cytohistology. From this study, biopsies appear more helpful to detect malignant cells than aspirates. Immunological staining clearly leads to a better definition of tumour cells in aspirates.