Increased plasma concentrations of YKL-40, also called chitinase-3-like-1 protein (CHI3L1), have been correlated with disease severity in a variety of malignant and inflammatory diseases. The objective of the current study was to assess pretransplant recipient and donor CHI3L1 polymorphisms and plasma YKL-40 concentrations as prognostic biomarkers in a cohort of 149 patients treated with hematopoietic cell transplantation (HCT) after nonmyeloablative conditioning for hematologic malignancies. Recipients with pretransplant YKL-40 concentrations above the age-adjusted 95th percentile (high) had higher relapse-related mortality (33% versus 18%, P = .04; hazard ratio (HR) = 4.41, P = .01), lower progression-free survival (38% versus 64%, P < .01; HR = 2.84, P = .01), and overall survival (42% versus 69%, P = .01; HR = 3.09, P = .01). Recipients transplanted with donors with high YKL-40 concentrations had an increased probability and risk of grade 2-4 acute graft-versus-host disease (aGVHD) (93% versus 62%, P < .01; HR = 2.25, P = .02). CHI3L1 polymorphisms were associated with plasma YKL-40 concentrations, but not with clinical outcomes. In conclusion, our study suggests that plasma YKL-40 could function as a biomarker for relapse risk and treatment-related toxicity, and possibly as a tool complementing clinical risk scores such as the HCT comorbidity index.