The objectives of this study were to investigate the development and pathogenesis of osteoporosis in stroke in-patients, to compare the bone mineral density (BMD) of the paretic and non-paretic sides, to study longitudinal changes during the period on the rehabilitation ward and to relate BMD to demographic, impairment and disability variables. Participants were 32 first-stroke in-patients (19 men), with an average age of 62.4 +/- 8.1 years. Demographic and clinical characteristics were documented. The BMD of lumbar spine, bilateral distal radius and femoral neck were measured and compared at admission and discharge. The associations between bone loss and age, sex, time since stroke, Brunnstrom motor recovery scores and functional independence measure motor (mFIM) scores were assessed. The mean percentage differences in BMD between paretic and non-paretic arms at discharge were 12% (P < 0.001) and 3.5% (P < 0.05), respectively, and between paretic and non-paretic legs were 5% (P < 0.01) and 2% (P < 0.05), respectively. There was a statistically significant correlation between BMD loss and Brunnstrom stage (r = -0.41) and mFIM score at admission (r = -0.42). In conclusion, patients with hemiparesis due to stroke are at increased risk of developing osteoporosis on the paretic side. Higher motor impairment and functional dependency at admission increases the risk. New strategies are needed for stroke patients to prevent further decreases in BMD and reduce the risk of fractures.