Donor lymphocyte infusion (DLI) can produce durable remissions in patients with chronic myelogenous leukemia (CML) who have a relapse after an allogeneic stem cell transplantation. However, the best modality to administer DLI is still unclear. The effect of the initial cell dose (ICD; ie, mononuclear cells x 10(8)/kg received in the first instance) on outcome was retrospectively analyzed in 298 of 344 patients treated with DLI at 51 centers. Patients were classified into 3 groups according to the ICD: 98 in group A (<or= 0.20), 107 in group B (0.21-2.0), and 93 in group C (> 2.0). Additional infusions were given to 62%, 20%, and 5% of patients in groups A, B, and C, respectively. A lower ICD was associated with less graft-versus-host disease (GVHD; A, 26%; B, 53%; C, 62%; P <.001), less myelosuppression (A, 10%; B, 23%; C, 24%; P =.01), and similar response rate (A, 78%; B, 73%; C, 70%; P =.48). Nonadjusted estimates of 3-year survival, failure-free survival, and DLI-related mortality were 84%, 66%, and 5% respectively, in group A; 63%, 57%, and 20% in group B; and 58%, 45%, and 22% in group C. Outcome analysis was adjusted for patient age, donor type, sex of donor, sex mismatch, disease phase at transplantation, T-cell depletion, interval from transplantation to DLI, GVHD prior to relapse, relapse type, and date of DLI. After adjustment, lower ICD was associated with less GVHD, less myelosuppression, same response rate, better survival, better failure-free survival, and less DLI-related mortality. Our results suggest that the first DLI dose should not exceed 0.2 x 10(8) mononuclear cells/kg.