Summary 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.