Serum concentrations of mannan-binding lectin (MBL) were determined in the sera of 67 HIV-seropositive patients in different stages of HIV disease and in the sera of 75 HIV-seronegative healthy individuals. In the asymptomatic (AS) HIV-infected persons MBL concentrations were found to be significantly (P < 0.05) lower than in the HIV-seronegative controls, whereas in the AIDS patients they were not. Very low (< or = 25 ng/ml) MBL serum concentrations were detected in 5/19 (26.3%) and 7/75 (9.3%) of the AS HIV-seropositive and HIV-seronegative individuals, respectively (P = 0.06). In the sera of the HIV-infected patients, MBL levels positively correlated to the neopterin concentrations (Spearman correlation coefficient, 0.401, P = 0.0009) while they negatively correlated to the percentage (-0.447, P = 0.0011) and absolute number (-0.453, P = 0.0012) of the CD4+ lymphocytes. These observations indicate that MBL level, which is under strict genetic control, may influence the susceptibility to HIV infection and the progression of HIV disease.