To determine the distribution of major blood lymphocyte subsets we evaluated blood lymphocytes by flow cytometry in adenovirus-infected infants aged 30-730 d. In addition, interleukin-1-receptor antagonist, interleukin-10 and transforming growth factor-beta1 were measured in serum by enzyme-linked immunosorbent assay. According to clinical parameters, mechanical ventilation and outcome, infections were classified as moderate (n = 15), severe (n = 11) and fatal (n = 12). Controls were 13 healthy children. In severe and fatal infection, T cells (CD5+/CD19-), NK effectors (CD16+), CD4+ T subset and B1 subset of B lymphocytes (CD5+/CD19+) were all significantly decreased. CD8+ cells were decreased in severe but not fatal cases. There was no difference in serum values of interleukin-10; however, fatal cases had high interleukin 1-receptor antagonist values. Interestingly, patients with moderate infection showed significantly increased values of transforming growth factor-beta1. These results demonstrate that life-threatening adenoviral infection is associated with marked abnormalities in blood lymphocyte and cytokine profile.