To elucidate the impact of infused CD34+ cell doses on transplant outcome, we retrospectively analyzed 851 adult patients who received peripheral blood stem cell transplantation (PBSCT) from human leukocyte antigen (HLA)-matched related donors. The patients were divided into high- and low-CD34 groups at the cutoff value of 4.5 × 106/kg. Overall, the high CD34 group showed early neutrophil and platelet recovery. Stratification of disease risks demonstrated that among the patients with low-risk diseases, the high-CD34 group showed better disease-free survival (DFS) (64.9% vs. 55.5%, P = 0.0415) than did the low-CD34 group, without any increase in graft-versus-host disease (GVHD). Meanwhile, a higher CD34+ cell dose had no impacts on the outcomes of patients with high-risk diseases. Multivariate analyses for the patients with low-risk diseases revealed that a high CD34+ cell dose (hazard ratio [HR] 0.72, P = 0.048) and development of grade III-to-IV acute GVHD (HR 1.64, P = 0.018) were significantly associated with DFS. An excessive dose of CD34+ cells (>8.0 × 106/kg) led to an increase in acute GVHD. By stratification of disease risk, a CD34+ cell dose between 4.5 and 8.0 × 106/kg can be suggested for patients with low-risk diseases who undergo PBSCT from HLA-matched related donors.