Venous thromboembolism (VTE) remained a major complication in patients undergoing hematopoietic cell transplantation (HCT) without established risk assessment models (RAMs) to guide thromboprophylaxis. Researchers conducted a temporal and external validation investigation of the newly developed HIGH-2-LOW RAM to resolve this issue. They chose adult allogeneic HCT patients from the Fred Hutchinson Cancer Research Center (FHCRC) and the MD Anderson Cancer Center (MDACC). Patients who died got anticoagulation or failed to engraft platelets by day 30 were excluded from the study. By day 180, the primary outcomes were defined as total VTE and pulmonary embolism ± lower extremity deep venous thromboembolism (PE/LE-DVT). Covariates were weighted according to the original model, substituting grade 2–4 GVHD for grade 3–4. Investigators evaluated discrimination and calibration. Included were 765 patients from FHCRC and 954 patients from MDACC. On day 180, the incidence of VTE was 5.1% at FHCRC and 6.8% at MDACC. The HIGH-2-LOW score at FHCRC had a c-statistic of 0.67 (0.59–0.75) for VTE and 0.75 (0.64–0.81) for PE/LE-DVT, but at MDACC, the c-statistics were 0.62 (0.55–0.70) for VTE and 0.70 (0.56–0.83) for PE/LE-DVT. About 25% and 23% of patients in the 2 cohorts were classified as high risk (2+ points). High risk versus low risk was linked with odds ratios (OR) of 2.80 (1.46–5.38) for VTE and 4.21 (1.82–9.77) for PE/LE-DVT at FHCRC, and ORs of 3.54 (2.12–5.91) for VTE and 6.82 (2.30–20.27) for PE-LE-DVT at MDACC. The HIGH-2-LOW RAM identified allogeneic HCT participants at high risk for VTE in both validation cohorts. It could enhance evidence-based thromboprophylaxis post-transplant decision-making.