A multicentre study was performed to compare clotting factor use and outcome between on-demand and prophylactic treatment strategies for patients with severe haemophilia. Data on treatment and outcome of 49 Dutch patients with severe haemophilia, born 1970-80, primarily treated with prophylaxis, were compared with those of 106 French patients, who were primarily treated on demand. Dutch patients received intermediate dose prophylaxis, for a median duration of 12.7 years. Patients primarily treated with prophylaxis had fewer joint bleeds per year (median 2.8 vs. 11.5), a higher proportion of patients without joint bleeds (29% vs. 9%), lower clinical scores (median 2.0 vs. 8.0), and less arthropathy as measured by the Pettersson score (median 7 points vs. 16 points). Mean annual clotting factor use was equal at 1,488 +/- 783 IU kg-1 year-1 (mean +/- standard deviation) for patients primarily treated with prophylaxis and 1,612 +/- 1,442 IU kg-1 year-1 for patients primarily treated on demand. These findings suggest that, compared with a primarily on-demand treatment strategy, a primarily prophylactic treatment strategy leads to better outcome at equal treatment costs in young adults with severe haemophilia.