We have reported previously that allergen-specific serum IgE levels were correlated with allergen-induced interleukin (IL)-4 in type I allergic individuals. Here, we wanted to investigate whether IL-13, another switch factor for IgE, was induced by allergen in vitro and if so, whether this was correlated with the elevated serum IgE-levels seen in atopic individuals, and whether the cytokine profile changed during pollen season. Peripheral blood mononuclear cells from 14 atopic and 14 healthy individuals collected out of the pollen season were incubated in vitro with allergens (birch or timothy) and the number of IL-4, IL-13, IL-10 and IFN-γ producing cells was determined by ELISPOT. In response to the specific allergen, IL-13-producing cells were seen in allergic but not in healthy individuals. The number of IL-13-producing cells was significantly correlated with the allergen-specific serum IgE levels. When the allergic individuals were tested during the pollen season, the number of allergen-specific IL-4- and IL-13-producing cells, as well as serum levels of specific IgE, increased. The IL-13 increase seen in ELISPOT was confirmed by a RT-PCR assay. No seasonal changes were seen in response to purified protein derivative (PPD) or the mitogen PHA. During the pollen season, the IL-4 and IL-13 responses were highly correlated. Taken together, our results support the roles of both IL-13 and IL-4 in the regulation of allergen-specific IgE levels in atopic individuals.