We tested the independent prognostic impact of 2 commonly used biomarkers, C-reactive protein (CRP) and interleukin (IL)-6, on the outcomes of allogeneic hematopoietic cell transplantation (HCT). Consecutive patients who underwent a uniform reduced-intensity conditioning (RIC) regimen of fludarabine (Flu), melphalan (Mel), and alemtuzumab were evaluated retrospectively. Cryopreserved serum samples drawn before the RIC were available to measure CRP levels in 81 patients and IL-6 levels in 79 patients. Patients with CRP levels above the median of 18.5 mg/L had significantly more grade 3-4 hepatic toxicity (P=.01), longer HCT hospital stay (P=.005), more acute graft-versus-host disease (aGVHD) (P=.003), greater nonrelapse mortality (NRM) (P=.01), and inferior overall survival (OS; P=.02). Higher baseline CRP showed no significant correlation with grade 3-4 infectious toxicity (P=.09). In contrast to CRP, pre-HCT IL-6 levels above the median of 78.3 pg/mL did not confer a statistically significant increased risk of toxicity or mortality. An elevated HCT comorbidity index (HCT-CI) did not predict for any measure of HCT morbidity. After adjustment for disease status, comorbidity, performance status, and age, elevated CRP concentration remained predictive of NRM. These data require confirmation in non-T cell-depleted conditioning regimens. If validated, they suggest that preconditioning CRP holds promise for enhancing estimates of transplantation tolerance.