The NIH Chronic Graft-versus-Host Disease (cGVHD) Consensus Project Ancillary and Supportive Care Guidelines recommend annual assessment of bone mineral density (BMD) to monitor bone health. The study of osteoporosis in patients with cGVHD has been limited to small numbers of patients and the guidelines are based on experiences in other chronic diseases and expert opinion. We hypothesized that the prevalence of osteoporosis is high in a cohort of 258 patients with moderate to severe cGVHD due to prolonged exposure to risk factors for osteoporosis after allogeneic hematopoietic stem cell transplantation. We defined osteoporosis using BMD criteria (T-score ≤ -2.5) at three anatomical sites (femoral neck – FN, lumbar spine – LS, total hip – TH) and characterized risk factors through univariate and multivariate analyses. We found that low body weight (FN p<0.0001, LS p=0.0002, TH p<0.0001), malnutrition (FN p=0.0002, LS p=0.03, TH p=0.0076), higher platelet count (FN p=0.0065, TH p=0.0025), higher average NIH organ score (FN p=0.038), higher prednisone dose (LS p=0.032), lower complement component 3 (LS p=0.0073), and physical inactivity (FN p=0.01) were associated with osteoporosis in one or more site. T-scores were significantly lower in the FN than in the other two sites (p<0.0001 for both). The prevalence of osteoporosis and osteopenia was high (17% and 60%, respectively), supporting current recommendations for frequent monitoring of BMD. The association of higher platelet count in cGVHD patients with osteoporosis has not been previously reported and presents a new area of interest in the study of osteoporosis after allogeneic hematopoietic stem cell transplantation.