The cutaneous symptoms in non-immediate reactions to drugs are not always clinically distinguishable from those induced by viruses, especially during the early phase of the reaction. Moreover, viral infections and drug reactions often coexist and identification of the etiological agent is necessary. Discerning the differences in the immunological response between both may help in the diagnosis. The aim of this study was to determine possible differences in the immunological response in non-immediate cutaneous reactions to drugs versus cutaneous viral-induced diseases in children. Two groups of children were evaluated: one with non-immediate drug-induced cutaneous reactions (DICR) and another with virus-induced cutaneous reactions (VICR). A third group of children taking the same drugs as the DICR group and with no cutaneous disease or viral infections was included as controls. The lymphocyte markers CD3, CD4, CD8, CD16, CD19, CLA, CD25, CD69, CD45RO, CD45RA were determined by flow cytometry. IL-2, IL-4, IL-5, IFN-gamma, TNF-alpha and IL-10 mRNA were measured by RT-PCR. Data were compared by non-parametric and chi(2) statistical analysis. In DICR group (n=8) the diagnosis was established by temporal association, improvement after drug withdrawal, patch testing, and in some cases by controlled administration. All patients in the VICR group (n=10) were diagnosed based on a positive viral serology: the presence of IgM antibodies or seroconversion of IgG antibodies. There were significant differences between the three groups in peripheral lymphocytes expressing the skin homing receptor CLA (P < 0.01), the early activation marker CD69 (P < 0.001), and the memory (CD3+CD45RO+) (P < 0.02) and naive (CD3+CD45RA+) (P < 0.03) T cell subsets. Children with DICR showed a TH1 mRNA cytokine pattern whereas those with VICR showed a TH0 pattern. In lymphocyte subpopulations from children, differences in the immunological response between DICR and VICR can be detected in the expressions of the activation marker CD69 and the cutaneous homing receptor CLA, and in cytokine mRNA profiles.