A study on 315 patients undergoing transplantation with CD34+ selected blood cells from HLA-identical siblings was performed to determine risk factors for acute GVHD (aGVHD). Recipients of a dose of CD34+ cells (x 10(6)/kg) of 2 or less, more than 2 to 4, and more than 4 had a cumulative incidence of aGVHD grades I-IV of 21%, 35%, and 43%, respectively (log-rank P =.01); similarly, recipients of a dose of CD3+ cells (x 10(6)/kg) of 0.05 or less, more than 0.05 to 0.1, and more than 0.1 had a cumulative incidence of aGVHD grades I-IV of 18%, 35%, and 44%, respectively (log-rank P =.007). Using a Cox regression model, 4 independent factors for aGVHD I-IV were identified: increased CD34+ cell dose (P =.02), increased CD3+ cell dose (P =.02), female patients (P =.01), and higher patient age (> 42 years) (P =.007). This study shows, for the first time in T-cell-depleted transplantations, a positive correlation between the number of CD34+ cells and aGVHD and, also, that the number of CD3+ cells necessary to initiate aGVHD is lower than previously reported.