In order to clarify the association between hyperglycemia during the early period after allogeneic stem cell transplantation (allo-SCT) and adverse outcomes, we retrospectively analyzed 563 consecutive patients who underwent allo-SCT at our institute between 2008 and 2015. Patients were categorized into three groups according to mean fasting blood glucose levels on days 0-7 (normoglycemia group < 110 mg/dL, n = 347; mild hyperglycemia group 110-149 mg/dL, n = 192 and moderate/severe hyperglycemia group ≥ 150 mg/dL, n = 24). The median follow-up was 2.7 years. Patients in the moderate/severe hyperglycemia group had significantly worse characteristics. The cumulative incidences of 2-year non-relapse mortality (NRM) and the probabilities of 2-year overall survival (OS) in the normoglycemia, mild hyperglycemia and moderate/severe hyperglycemia groups were 7.5%, 19% and 29%, respectively (P < 0.01), and 69%, 53% and 33%, respectively (P < 0.01). In multivariate analyses, hyperglycemia was an independent predictor of high NRM (vs normoglycemia; mild hyperglycemia, hazard ratio (HR) 2.56, 95% confidence interval (CI) 1.56-4.18; moderate/severe hyperglycemia, HR 4.46, 95% CI 1.92-10.3) and poor OS (vs normoglycemia; mild hyperglycemia, HR 1.54, 95% CI 1.14-2.07; moderate/severe hyperglycemia, HR 1.61, 95% CI 0.89-2.91). In conclusion, hyperglycemia on days 0-7 after allo-SCT was associated with inferior outcomes.