A detailed clinical and radiological examination of the joints and laboratory studies were carried out on 139 haemophiliacs attending a single centre. The group included more patients with mild and moderate haemophilia (factor levels 6 to 60 per cent) than in previous studies. Haemarthrosis, the most common bleeding manifestation, had affected more than two-thirds of patients including many with mild or moderate disease. Restriction and contracture of the knees and elbows were the most common clinical features and, with the ankles, these joints were most frequently affected both clinically and radiologically. Using a combination of clinical and radiological features, 42 per cent of the patients could be classed as having 'definite' and a further 14 per cent 'possible' haemophilic arthritis. Although haemarthroses were equally prevalent in patients with classical haemophilia and Christmas disease, arthritis was more frequently present in the former. Haemarthrosis and joint disease were exceptional in von Willebrand's disease. The prevalence of arthritis generally related to disease severity as measured by factor level but, in contrast to earlier studies, definite arthritis was seen in some patients with factor levels up to 20 per cent of normal although the number of affected joints was less in these patients with milder disease. Laboratory test abnormalities including circulating immune complexes and hypocomplementaemia were noted in some patients but the abnormalities correlated poorly with clinical features. The present results suggest a recent slight reduction in the prevalence or severity of haemophilic arthritis, possibly attributable to recent improvements in factor replacement treatment. Longer-term studies are required to show whether arthritis is indeed lessening or whether the onset is merely being delayed.