During the 1960s, it was reported from Sweden that haemophiliacs with factor levels above 1% rarely develop arthropathy. This observation suggested that severe haemophilia could be converted to a milder form by regular infusions with factor concentrate. After several earlier publications, a report was published in 1992 that detailed 25 years' experience with prophylaxis in 60 patients from the Malmö centre. The results showed that starting prophylaxis early in life with a dose regimen that would prevent factor VIII or IX plasma levels from falling below 1% could prevent the development of haemophilic arthropathy. Also, older age groups who had received less intensive treatment, and who started prophylaxis later in life, were still in a much better condition than historic controls. In the 1970s several small but well-controlled studies from the USA, Germany and Italy clearly showed the benefit of prophylaxis in reducing bleeding frequency. Early experience from the Netherlands was published in 1971. Since these early studies, the results have been corroborated from many countries and in a large multinational study. Although the benefits of prophylaxis seem unquestionable, several research questions remain to be better elucidated, such as when to start and when to stop, dosing and dose interval, and how to assess the long-term treatment effects. These issues are of great economic importance, and the need for health economical studies is obvious.