To evaluate whether vertical HIV infection interferes with the expression of CD28 on T lymphocytes, 25 HIV-infected children and 29 seroreverted children born to HIV+ mothers were studied. The percentage of CD28- cells among CD8+ T lymphocytes was higher in HIV-infected children than in controls (P < 0.001). In fact, in HIV-infected children, this percentage was elevated from the first year of life, while in healthy seroreverted children, the proportion of CD28- cells among CD8+ cells rose progressively with age (r = 0.49; P = 0.008). In HIV+ children, the CD8+ CD28-, but not CD8+ CD28+ cell proportion was significantly correlated with immunological markers of disease progression, such as CD4+ cell loss (r = -0.65; P < 0.001) and the level of in vitro spontaneous lymphocyte apoptosis (r = 0.53; P = 0.03).