A survey of prevalence of anti-HIV in patients with haemophilia A, B or von Willebrand's disease in the UK was carried out in July-August 1986 and 86 of the 109 centres contacted sent in information. Of 2330 haemophilia A patients, 955 (41%) were found to be anti-HIV positive; 26 (6%) out of 401 haemophilia B patients were found positive. The prevalence in severely affected patients was greater-59% in haemophilia A patients tested and 11% in haemophilia B patients tested. Comparison with a similar survey carried out by the Haemophilia Centre Directors in 1985 showed that 38 patients with haemophilia A and seven with haemophilia B had seroconverted since the 1985 survey. In only one of the haemophilia A patients was it possible to say that this seroconversion had probably been caused by heated concentrate.