Markers of immune function present before infection may determine the subsequent course of disease in HIV-infected individuals. In 1983, we measured immune function in a group of haemophiliacs in Edinburgh. In 1984, 18 of these patients became infected with HIV-1 from contaminated factor VIII. We have followed-up these patients since their seroconversion. The rate of disease progression, as assessed by the appearance or not of AIDS symptoms or signs within five years of seroconversion, was related both to the concentration of total plasma IgM before exposure to infection and to the pattern of specific IgM and IgA anti-HIV response around the time of IgG seroconversion. Disease progression also correlated with concentrations of plasma interleukin-2 receptor (a marker of lymphocyte activation) and with the number and percentage of circulating DR + ve (activated) T cells. Our findings show that the extent of host immune reactivity, which may be genetically determined, is a powerful factor in the pathogenesis of HIV-associated disease.