The cellular immune response to respiratory syncytial virus (RSV) infection was examined in infants aged 1-21 months who were hospitalized because of RSV infection or non-RSV-related illness. RSV- or control-stimulated peripheral blood mononuclear cells were examined to determine RSV-specific intracellular T helper-1 (Th1) and T helper- 2 (Th2) cytokine expression, chemokine messenger RNA (mRNA) expression, and cell surface markers. Patients hospitalized because of RSV infection had increased numbers of CD16(+) and CD56(bright) cells and had RSV-specific increases in Th1 (interleukin [IL]-2 and interferon-gamma) and Th2 (IL-4 and IL-6) cytokines and CC chemokines (macrophage inflammatory protein [MIP]-1alpha, MIP-1beta, and RANTES [regulated on activation, normally T cell expressed and secreted]) mRNA expression. The results suggest that RSV infection induces both Th1 and Th2 cytokine expression and CC chemokine expression.