The chemokine receptor CCR5 is an important co-receptor for cell fusion. A 32-bp deletion of the CCR5 gene, leading to complete absence of functional CCR5 expression, has been associated with resistance to human immunodeficiency virus (HIV) infection in homozygotes and slower HIV disease progression in heterozygotes. The objectives of this study were to assess the effects of this 32-bp deletion on transmission of HIV infection and on HIV disease progression in haemophilic individuals. Six HIV-negative patients from our centre, known to have been exposed to infectious factor VIII concentrates, have been analysed. Three of these patients possess the CCR5 32-bp deletion, two patients being homozygous. The presence of the CCR5 32-bp gene deletion has also been analysed in 71 HIV-positive patients. In this group of patients, there was a lower than expected incidence of the 32-bp deletion. Those who possess the 32-bp deletion progress to AIDS more slowly than those who do not (P = 0.05, log-rank test). Rates of CD4 loss were slower in those heterozygous for the gene deletion. We confirm that heterozygosity for the 32-bp gene deletion in CCR5 is partially protective against initial infection with HIV. In those heterozygous patients who became infected with HIV, disease progression was slower.