Allogeneic hematopoietic cell transplantation (HCT) continues to be associated with substantial rates of nonrelapse mortality (NRM). Numerous factors influence glucose metabolism among HCT recipients. We hypothesized that "malglycemia," defined as hyperglycemia, hypoglycemia or increased glycemic variability, is associated with increased mortality in HCT patients. In a retrospective cohort study Cox regression was used to assess the association of malglycemia after transplant with day 200 NRM. A total of 66,062 blood glucose (BG) measurements from 1175 adult allogeneic HCT recipients between 2000 and 2005 at the Fred Hutchinson Cancer Research Center were evaluated (median 0.55 values per patient-day, range: 0.09-3.62). Overall, there were 215 cases of NRM by day 200 post-HCT and 601 deaths from any cause throughout observation. After adjustment for previously identified factors associated with NRM, all 3 components of malglycemia were associated with increased NRM when individually modeled as time-dependent covariates. Specifically, the hazard ratio for death was 1.93 for BG >200 mg/dL (P = .0009) and 2.78 for BG >300 (P = .0004) compared with BG 101-150 mg/dL. A minimum BG </=89 was associated with a risk of day 200 NRM 2.17 times that of a minimum BG >89 (P < .0001). The upper quartile of glucose variability was associated with a 14.57-fold increase in risk of NRM by day 200 relative to the first quartile (P < .0001). These retrospective data indicate that malglycemia is associated with mortality following HCT. The applicability of these findings to other situations and whether correcting malglycemia in HCT can lead to reductions in mortality remain to be determined.