The immunological mechanisms by which respiratory syncytial virus (RSV) contributes to the development of asthma are poorly understood. gammadelta T cells are important in mucosal defence, and may contribute to the establishment of primary immune responses by producing cytokines early during respiratory infections. Thus, we used flow cytometry and intracellular cytokine staining to investigate the expression of interferon (IFN)-gamma and interleukin (IL)-4 by mitogen-stimulated gammadelta T cells from the peripheral blood of 15 hospitalized infants with RSV bronchiolitis, seven rotavirus-infected infants and eight normal controls. gammadelta T cells from RSV-infected infants had a lower proportion of IFN-gamma-producing cells (median, 4.00%; range, 0.58-6.60%) and a slightly but significantly higher proportion of IL-4-producing cells (median, 0.40%; range, 0.13-2.76%) than rotavirus-infected infants (median, 32.10%; range, 14.43-61.21%; P < 0.01, median, 0.00%; range, 0.00-0.00%; P < 0.05) in the acute phase. By contrast, differences in cytokine production by total CD3+ T cells did not differ significantly between patient groups. Thus, reduced IFN-gamma-production by gammadelta T cells in the peripheral blood of RSV-infected infants is accompanied by increased Th2 cytokine production during the acute phase of disease. At follow-up, eight children had recurrent episodes of wheezing. The frequencies of IFN-gamma-producing gammadelta T cells were significantly lower in patients who developed recurrent wheezing (median, 0.65%; range, 0.02-1.75%) than in patients without recurrent wheezing (median, 6.90%; range, 5.25-10.98%; P < 0.005). Cytokine production by gammadelta T cells may therefore be important in the pathogenesis of acute RSV disease, and play a part in the development of recurrent childhood wheezing after bronchilolitis.