The influence of busulfan (BU) plasma concentration on outcome of transplantation from HLA identical family members for the treatment of chronic myelogenous leukemia (CML) was examined in 45 patients transplanted in chronic phase (CP) (n = 39) or accelerated phase (AP) (n = 6). All patients received the same regimen of BU, 16 mg/kg orally and cyclophosphamide (CY), 120 mg/kg intravenously. Plasma concentrations of BU at steady state (C(SS)BU) during the dosing interval were measured for each patient. The mean C(SS)BU was 917 ng/mL (range, 642 to 1,749; median, 917; standard deviation, 213). Of patients with C(SS)BU below the median, seven (five of 18 in CP and two of four in AP) developed persistent cytogenetic relapse and three of these patients died. There were no relapses in patients with C(SS)BU above the median. The difference in the cumulative incidence of relapse between the two groups was statistically significant (P = .0003). C(SS)BU was the only statistically significant determinant of relapse in univariable or multivariable analysis. The 3-year survival estimates were 0.82 and 0.64 for patients with C(SS)BU above and below the median (P = .33). There was no statistically significant association of C(SS)BU with survival or nonrelapse mortality, although the power to detect a difference in survival between 0.82 and 0.64 was only 0.24, similarly C(SS)BU above the median was not associated with an increased risk of severe regimen-related toxicity. We conclude that low BU plasma levels are associated with an increased risk of relapse.