Neuropsychological findings from investigation of 46 HIV-seropositive asymptomatic and 14 HIV-seropositive symptomatic haemophiliacs without AIDS-related complex (ARC) or AIDS, with known duration of HIV seropositivity were compared with 29 seronegative controls. Subjects were assessed blindly using a battery of sensitive computerized neuropsychological tests. They underwent a thorough neurological examination, were assessed for mood and screened for psychopathology. Symptomatic HIV-positive haemophiliacs without ARC or AIDS showed statistically significant decreased performances compared with HIV-negatives in choice reaction, visuomotor coordination and global attentional performance (P = 0.018, 0.039 and 0.044, respectively). HIV-positive asymptomatic subjects gave lower performances than HIV-negative subjects in all tests, although these differences were not statistically significant. However, there was a statistically significant trend for these findings between seronegative, asymptomatic and symptomatic groups. Impairment was not associated with mood factors. Duration of seropositivity was found to be a more important factor than Centers for Disease Control stage in the choice reaction test (P less than 0.01). These findings indicate that mild cognitive impairment observed during the natural history of HIV infection in haemophiliacs without ARC or AIDS may be a progressive phenomenon not necessarily associated with the clinical expression of HIV infection.