We have developed a nonmyeloablative conditioning regimen that combines fludarabine, 3 x 30 mg/m², and 2 Gy total body irradiation to extend unrelated hematopoietic cell transplantation (HCT) to older patients or those with comorbid conditions. Postgrafting immunosuppression consisted of mycophenolate mofetil (MMF) and cyclosporine. After nonmyeloablative conditioning, the cells contained in the graft have been assumed to both create marrow space for donor engraftment and exert graft-versus-tumor effects. In this study, we retrospectively examined whether variations in cellular graft composition correlated with clinical outcomes in 130 consecutive pts given unrelated grafts for treatment of hematological malignancies. Median pt age was 54 (range 5–71) years. Fourteen pts received marrow and 116 G-GSF-mobilized peripheral blood cells (G-PBMC). Graft composition was determined by FACS analyses and expressed as cells/kg of recipient weight. Cell subsets analyzed were total nucleated cells (TNC), monocytes, CD34+, CD3+, CD4+, CD8+, CD3−CD56+, CD3+CD56+ and CD20+ cells. After adjusting for prior chemotherapy treatment, and recent autologous HCT, we found that higher numbers of donor monocytes (p=.003), CD3+ (p=.0007), CD4+ (p=.001), CD8+ (p=.004), CD3−CD56+ (p=.003), and CD34+ (p=.0001) cells were associated with higher levels of day 28 donor T-cell chimerism. When analyses were restricted to G-PBMC recipients, the association remained statistically significant for CD3+ (p=.03), CD4+ (p=.007), CD3−CD56+ (p=.04), and CD34+ (p=.01) cells. After initial engraftment, 6 of 14 marrow recipients and 8 of 116 G-PBMC (p=.001) recipients rejected their grafts. In multivariate analysis, higher numbers of donor CD3+ (p=.02), CD8+ (p=.005) and CD34+ (p=.01) cells were associated with a lower probability of graft rejection. Similar trends were observed in the G-PBMC recipients, although these associations lacked statistical significance, likely as a result of the small number of graft rejection events. Acute graft-versus-host disease (GVHD) of grades II, III and IV occurred in 71 (54.6%), 16 (12.3%) and 2 (1.5%) pts, respectively. Chronic GVHD requiring therapy was seen in 72 (55.4%) pts. After adjusting for pt age, HLA-compatibility, female donor to male recipient, CMV serostatus, malignancy, MMF bid vs tid and Charlson comorbidity scores at transplantation, none of the cell-type variables were significantly associated with acute or chronic extensive GVHD. Fifty-two pts (40%) relapsed/progressed after HCT, and 22 (16.9%) died because of nonrelapse causes. After controlling for autologous/allogeneic HCT, Charlson’s comorbidity index scores at transplantation, and disease risk, none of the cell variables was significantly associated with relapse/progression or nonrelapse mortality, though TNC dose was significantly associated with improved progression-free survival (p=.01); this association was no longer significant (p=.13) when the analysis was restricted to G-PBMC recipients. In conclusion, increasing the CD34+, CD3+ and CD8+ cell doses in the unrelated grafts after nonmyeloablative conditioning improved donor T-cell engraftment without increasing the risk of GVHD.