Using immunohistological techniques, the cellular composition of lymph nodes was assessed in 18 patients who had died 15 to 326 days after allogeneic bone marrow transplantation for leukaemia. The lymph nodes showed reduced cellularity of the cortex and paracortex, dilated sinuses and no lymphoid follicles. The majority of leucocytes were T lymphocytes with an inversion of the normal T4:T8 ratio. No cells were detected expressing immature cortical thymocyte antigens, using NA1/34 and OKT10, but an excess of T11 (E rosette receptor)⁺ cells over the sum of T4⁺, T8⁺ and HNK1⁺ cells raised the possibility of the presence of immature cells. B lymphocytes were extremely rare and present as clusters in only two patients. Despite this, plasma cells were prominent in many cases and their number increased with time post transplant. The predominant immunoglobulin heavy chain class was IgA in seven cases, IgG in three cases, IgM in two cases and IgE in one case with no relationship between dominant class and days post transplant. In patients with graft‐versus‐host disease (GvHD), there was a significantly lower T4:T8 ratio but no increase in expression of lymphocyte activation markers. Pyknotic leucocytes were present in half of the cases with GvHD and none of the other cases. No differences were detected in patients who had received marrow purged with monoclonal antibodies (Campath‐I or UCHT1). Chimeric studies on three recipients of one haplotype matched marrow, using a monoclonal antibody specific for HLA‐A2 and A28 antigens, showed a significant influx of donor cells by 56 days but this did not appear to be an immediate prelude to full morphological reconstitution.