Two hundred and eight serial bone marrow samples from 49 consecutively diagnosed children with neuroblastoma were studied retrospectively for evidence of tumour invasion. Bone marrow involvement was found in 24 patients at diagnosis and in four more at a later stage in their disease. Trephine biopsies were more effective than aspirates for tumour detection in 20% of the 154 paired aspirate/trephine procedures, whilst the reverse was the case in 7%. Imprints of trephines gave no additional information. Bilateral sampling (aspirates and trephines) improved the tumour detection rate by 10% over that attained by sampling a single site. There was some correlation between specific appearances in aspirate and in trephine samples, for example the hypocellular smear and the trephine biopsy showing much stromal reaction to tumour infiltration. Bilateral iliac crest bone marrow aspirates and trephine biopsies are indicated in children with neuroblastoma, both for initial staging and for monitoring of progress.