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 ≥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.