Two hundred and fifty-two consecutive patients with severe aplastic anaemia were conditioned by cyclophosphamide and given marrow transplants from HLA-identical siblings or parents. The results were analysed to examine the influence of marrow cell dose on graft rejection, speed of engraftment, survival and acute and chronic graft-versus-host disease (GVHD). Sixty patients received a corrected marrow cell dose below 2.2 x 10(8) nucleated cells per kg body weight, 73 patients between 2.2 and 3.2 x 10(8) cells/kg, 60 patients between 3.3 and 4.2 x 10(8) cells/kg, and 59 patients more than 4.2 x 10(8) cells/kg. The significant association between high marrow cell dose and low risk of graft rejection previously reported was confirmed. In addition, high marrow cell dose resulted in small but statistically significant reductions in time to granulocyte and platelet recoveries in patients with sustained engraftment. High marrow cell dose was also associated with a significant improvement in patient survival, a result solely due to the reduction in the incidence of graft rejection. Neither acute nor chronic GVHD were influenced by marrow cell dose. We conclude that the maximum marrow cell dose should be transplanted in patients with severe aplastic anaemia. Maximum marrow cell doses are likely to decrease the incidence of graft rejection, shorten the time to engraftment thereby decreasing the need for blood products and antibiotic support, and improve overall patient survival.