Although the nature of haemophilia has been understood for thousands of years, knowledge of its molecular genetics is recent. These X-linked bleeding disorders have diverse underlying DNA defects and, in 1992, DNA inversion within the X chromosome was found to explain half of the most serious cases of haemophilia A. The life-span and quality-of-life for patients with haemophilia had improved steadily throughout the early 1980s but the principal cause of death remained intracranial haemorrhage until the epidemic of HIV infection due to contaminated factor concentrates. Infection with hepatitis C virus is almost universal for patients treated with clotting factors before 1985. No curative treatment is available for hepatitis C at present. Knowledge of the transmission of viruses in concentrates has led to important developments in processing techniques to eliminate them. Recombinant technology has produced factor VIII and, more recently, factor IX concentrate which is likely to be very safe. Development of inhibitors to factor concentrates (especially factor VIII) remains one of the most serious complications of haemophilia. The variety of treatments available testifies to the lack of a single universally efficacious one. The use of prophylactic treatment has been conclusively demonstrated to result in a preservation of joint function in severely affected patients who might otherwise develop significant joint problems. The many facets of the care of patients with severe haemophilia, ranging from dental care to genetic counselling, can be advantageously co-ordinated in a haemophilia comprehensive care centre.