Bone marrow cellularity estimated by biopsy was compared to the cellularity of the aspirate particle smear and the volumetric method in two groups of children. In the first group, 101 consecutive bone marrow biopsies and aspirates were evaluated from patients with various diagnoses. In the second group, 20 patients with acute nonlymphoblastic leukemia were studied with 80 biopsies and aspirates at diagnosis and following chemotherapy. A wide discrepancy was noted between bone marrow cellularity confirmed by biopsy vs. the particle smear or the volumetric method in both groups. Neither the volumetric nor the particle method provides a good correlation of bone marrow cellularity. We also compared the volumetric method with that of the biopsy to evaluate the efficacy of the former method in detecting bone marrow infiltration by solid tumors. The volumetric method is an accurate modality of identifying solid tumor infiltration in the bone marrow.