To investigate mould effects on health, the concentration of mould spores in air and dust was determined during a prospective cohort study. Clinical outcome was estimated by questionnaires and determination of specific IgE antibodies and intracellular cytokine production of T cells. A significant association was observed between the incidence of respiratory tract infections and exposure to Penicillium spores. Moreover, Aspergillus exposure was found to be associated with allergic rhinitis or related symptoms. In addition, T cells of children exposed to Aspergillus showed a significantly lower content of TH1 cytokines (IFN-gamma, TNF-alpha, IL-2) producing cells. Our data suggest that mould exposure is associated with several effects on health, depending on the species involved.