The aim of our study was to evaluate clinical diagnoses in symptomatic persons exposed to aerosols from humidifiers or air conditioners. In addition, we tried to identify the causative antigens. Results of clinical investigations, including inhalation challenge tests, demonstrated a typical hypersensitivity pneumonitis (humidifier lung) in 9 persons and isolated systemic symptoms without significant changes in lung function and chest x-rays (humidifier fever) in 3 persons. Microbiological studies revealed a variety of fungi and bacteria in the water supplies of humidifiers and air conditioners at patients' workplaces. The detection of 4 members of the order Sphaeropsidales (Deuteromycotina), not previously associated with humidifier-induced diseases, is of special interest. By means of an improved polystyrene tube-immunoradiometric assay, high concentrations of IgG antibodies against extracts prepared from water of patients' humidifier systems were found in all cases. In addition, patients demonstrated low concentrations of IgG antibodies against thermophilic Actinomycetes, and usually also against various fungi, such as Alternaria tenuis, Aureobasidium pullulans, Penicillium notatum, Aspergilli, and fungi of the order Sphaeropsidales, which were isolated and cultured from humidifier water supplies. The much higher concentrations of antibodies against humidifier/air conditioner water extracts seem to result from sensitization to a variety of antigens from different fungi and bacteria. For in vivo and in vitro diagnostic tests in humidifier-induced lung diseases, we especially recommend using extracts from water systems installed at the corresponding workplaces.