The prevalence of obesity, an important risk factor for both cardiovascular disease and arthropathy, is strongly increasing in the general population, but data for the haemophilia population are scarce. Obesity may have a more profound effect on arthropathy and on cardiovascular disease in patients with haemophilia. To assess the prevalence of obesity in haemophilia patients and install adequate measures, if necessary. We performed a nationwide postal survey to measure the prevalence of overweight and obesity in Dutch haemophilia patients in 1992 (n = 980) and 2001 (n = 1066). A random sample of the Dutch male population served as the control group. In adult haemophiliacs, the prevalence of overweight (BMI 25-30 kg m(-2)) increased from 27% to 35% (95% CI 31.1-38.0) and the prevalence of obesity (BMI >/=30 kg m(-2)) doubled from 4% to 8% (95% CI 6.0-10.1), which was comparable with the general population. The increased prevalence of obesity in boys with haemophiliacs, which tripled in 10 years, is alarming. The increased prevalence of overweight and obesity in patients with haemophilia may have a profound effect on morbidity and quality of life of haemophilia patients by aggravating pre-existing arthropathy and predisposing aged patients to cardiovascular disease. Measures to prevent overweight in haemophiliacs are therefore urgently needed.