To study early haematopoietic reconstitution after bone marrow transplantation bone marrow biopsy specimens taken in the third week after transplantation were evaluated. Cellularity was highly variable; localisation of the various cell lineages and the ratios of myeloid cells to erythroid cells were abnormal. Clustering of cells of the same lineage in the same stage of maturation was prominent. The bone marrow stroma showed many anomalies, including increased fibre content, periodic acid Schiff positivity of fat cells, oedema, sinus ectasia and granulomas. A comparison of biopsy findings with clinical and laboratory data showed a correlation between the amount of erythroid cells and the day of appearance of reticulocytes, as well as the number of reticulocytes. Absence of clustering of haematopoietic cells in four of five patients was associated with either failure of engraftment or early leukaemic relapse. Variables such as infections and administration of possibly myelosuppressive drugs did not influence bone marrow biopsy findings.