To determine whether the predominant in vivo activation of Th1 subset or Th2 subset in peripheral blood occurs in patients with allergic asthma, we measured serum levels of Th1 cell-derived interferon-gamma (IFN-gamma) and Th2 cell-derived interleukin-4 (IL-4). In addition, serum levels of soluble interleukin-2 receptor (sIL-2R) released from activated T cells were measured to determine a potential source of these cytokines. The elevation of serum IL-4 levels and nonelevation of serum IFN-gamma levels were observed in patients with allergic asthma compared with normal control subjects. A significant correlation between serum IL-4 levels and serum sIL-2R levels was observed in these patients. These results suggest that there is allergen-induced in vivo activation of Th2-like cells in peripheral blood of patients with allergic asthma.