Abstract Haploidentical hematopoietic cell transplantation (haplo‐HCT) with posttransplant cyclophosphamide (PTCY) is utilized for patients with hematological disorders but without conventional donors. The effects of new‐onset posttransplant diabetes mellitus (PTDM) following haplo‐HCT are unknown. We examined PTDM incidence and outcomes after haplo‐HCT with PTCY. Patients without diabetes receiving haplo‐HCT (n = 64) were analyzed for PTDM diagnosis (defined as blood glucose ≥ 200 mg/dL). By day 100, 14 (22%) patients developed PTDM (median, 18 days). Hyperglycemia (blood glucose ≥ 200 mg/dL) preceded corticosteroids in 11 (79%) individuals. PTDM patients had increased death/relapse (P = .029). PTDM occurs frequently, precedes corticosteroids, and leads to inferior outcomes following haplo‐HCT. PTDM prophylaxis/treatment may improve HCT survival.
