Reduced bone mineral density (BMD) in childhood is a risk factor for osteoporosis in later life. This case-control study determined the prevalence of low BMD, calcium intake and physical activity in 62 haemophilic children and 62 sex-, race- and age-matched healthy boys as controls. Lumbar spine (L2-L4) BMD was determined by dual-energy X-ray absorptiometry; BMD was considered to be low when Z-score > or =2. Physical activity was assessed using a validated questionnaire and calcium intake with a standardized quantitative food frequency questionnaire. Twenty-four patients (38%) had low BMD, whereas this was found in only 10 (16%) controls [odds ratio (OR) 2.86, 95% confidence interval (CI) 1.17-7.41; P = 0.014]. Lumbar BMD was significantly lower in the haemophilia patients than the controls (-1.6 +/- 1.0 vs. -0.9 +/- 0.9 respectively; P = 0.0004). Sedentary and low-grade exercise predominated in haemophilia (77%) versus control (50%) (OR 3.2, 95% CI 1.36-7.79; P = 0.003). There were no differences between groups with regard to calcium intake. Our results suggest that low-physical activity is a risk factor for reduced lumbar bone mass in the haemophilic group. This factor must be monitored to avoid a significant reduction in BMD that might contribute to further skeletal fragility.