Low CD34 + cell doses increase allograft-related mortality and very high doses increase the risk of graft-versus-host disease. The optimum CD34 + cell dose remains undefined. The effect of the CD34 + cell dose based on ideal weight was analyzed in 130 patients with hematologic malignancies undergoing reduced-intensity allogeneic blood cell transplantation in the context of factors known to affect the outcome: chemosensitivity, donor age, lactate dehydrogenase (LDH), human leukocyte antigen (HLA) match, performance status, and platelet count. The survival of patients receiving >8 x 10(6)/kg CD34 + cells was not significantly different from those receiving <6. The outcome of those receiving 6-8 x 10(6)/kg CD34 + cells was significantly better than the rest. This superiority was confirmed in multivariable analysis. Among patients receiving <or=8 x 10(6)/kg CD34 + cells, an increasing number of infused cells was associated with higher overall survival in a continuous fashion (Risk ratio (RR) 0.8759; p = 0.045). Cell dose based on actual weight did not correlate with survival. The number of CD34 + cells infused, a potentially modifiable factor, affects survival after reduced-intensity allogeneic transplantation. We recommend a CD34 + cell dose of 6-8 x 10(6) per kg ideal body weight to optimize outcome. The possible adverse effect of higher cell doses (>8) needs further confirmation.