OBJECTIVE To analyse haemopoietic regrowth and residual disease in bone marrow trephine biopsy specimens after treatment for acute leukaemia, using immunohistochemical staining. METHODS Biopsy specimens before and after treatment were studied from patients diagnosed as having acute myeloid or lymphoblastic leukaemia. Specimens after treatment encompassed periods from two to 56 weeks from the start of treatment. Routine haematoxylin and eosin and Giemsa stained sections were evaluated in association with immunostained preparations. A panel of antibodies was used, which reacts with epitopes showing restricted expression dependent on the lineage or maturation stage of cells. Results were evaluated in the light of clinical, peripheral blood, and marrow aspirate findings. RESULTS The speed and sequence of regrowth of haemopoietic cells were more variable than expected. Immunostaining highlighted features of dysplasia after treatment and in some cases assisted detection of residual or relapsed leukaemia. Peripheral blood and aspirate cell counts reflected accurately the amount of regrowth, but not the dysplasia, seen in biopsy samples. Delayed regrowth was associated with complex individual factors. CONCLUSIONS Morphological and immunohistochemical study of trephine biopsy specimens from patients treated for acute leukaemia provides information complementary to that obtained from peripheral blood and aspirated marrow. Variation in the timing and sequence of regrowth is highlighted. Immunostaining can aid in the detection of relapse or minimal residual leukaemia. The clinical relevance of dysplastic changes in biopsy specimens after treatment is uncertain, but such changes may persist for long periods.