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A cohort of 111 HIV-infected haemophiliacs has been followed for up to 11 years, during which time 33 patients have been diagnosed with AIDS. Twenty-seven of the cohort developed detectable p24 antigenaemia while remaining free of AIDS. These patients experienced an increased risk of progression to AIDS compared with those patients who were persistently p24-negative (relative risk 7.24; P < 0.0001, Cox proportional hazards model). The relative risk was reduced to 5.42 (P < 0.0001) after adjustment for age and cytomegalovirus seropositivity. After adjustment for the patients' declining CD4 lymphocyte count during follow-up, the relative risk fell dramatically to 1.97 and became non-significant (P = 0.2). p24-antigenaemic patients tended to develop AIDS at levels of similar CD4 lymphocyte counts to those who were persistently p24-antigen-negative (median CD4 lymphocyte counts, 70 and 50 × 106/I, respectively). These results suggests that the association between p24 antigenaemia and the rate of progression to AIDS can be explained largely by a more rapid decline in CD4 lymphocyte count among patients with p24 antigenaemia than in those without. The major pathological effects of increased plasma viral load, as detected by the presence or absence of p24 antigenaemia, appear to act via progressive CD4 lymphocyte depletion.