Twenty-seven children with severe haemophilia receiving regular prophylactic factor concentrate were evaluated to examine the overall effectiveness of prophylaxis in modern haemophilia care. The median age at the start of prophylaxis was 6.2 years (range 1.3-15.9 years) and the cumulative length of follow-up was 808 months (mean 30, rang 7-76 months). Nine patients required a central venous catheter for venous access (age range 1.3-5.2 years), eight boys could cannulate themselves and in 10 the parents performed regular venepuncture. The mean dose of concentrate given at the time of study was 31.8 U/kg three times weekly (range 12.5.6 U/kg) or 4900 U/kg/year (range 1900-8200). None developed an inhibitor on prophylaxis, though four had previously had an antibody. The median average annual number of bleeds in the 27 patients prior to prophylaxis was 14.7 (range 3.7-35.4). On prophylaxis this fell to 1.5 (range 0-12.5) (P<0.001) and in the group as a whole the frequency of bleeds diminished in successive years on prophylaxis. All 20 children with evidence of arthropathy improved on prophylaxis and eight had reversal of chronic damage such that their joints appeared normal at the time of study. There were reductions in the need for walking aids, in hospital admissions, and in numbers of school days lost for bleeding episodes. All families feel that prophylaxis has brought about an improvement in quality of life.