In order to study the influence of bone marrow CD34(+) cell dose on the outcome of allogeneic bone marrow transplantation (BMT), we analysed the results of BMT from HLA-identical siblings donors in 50 patients with chronic myeloid leukaemia (CML). The median numbers of nucleated cells (NC) and CD34(+) cells infused were 2.18 x 10(8)/kg (0.05-4.14 x 10(8)/kg) and 3.12 x 10(6)/kg (0.35-8.52 x 10(6)/kg), respectively. All patients engrafted. In univariate analysis, there was no correlation between the number of CD34(+) cells infused and the time to neutrophil recovery (P = 0.17). The Kaplan-Meier estimate of grade II-IV acute graft-versus-host disease (GVHD) at day 100 was 53 +/- 14% and 2-year survival was 46 +/- 15%. A number of CD34(+) cells infused greater than the median was the main factor increasing survival (P = 0.0006) and decreasing 100 day transplant-related mortality (P = 0.009). Patient-, disease- and transplant-related characteristics were not statistically different among patients receiving more or less than the median number of CD34(+) cells. The rate of infectious deaths was significantly higher in patients receiving less than 3.12 x 10(6) CD34/kg (48% vs 16%, P = 0.01). In a multivariable analysis, two factors associated with increased risk of death were advanced disease status at transplant (HR: 2.5 (95% CI: 1.09-5.75), P = 0.03) and a lower number of marrow CD34(+) cells infused/kg (HR: 4.55 (95% CI: 1.87-10.90), P = 0.0008).