We describe a case report of one patient with hypersensitivity pneumonitis (humidifier lung) due to exposure to contaminated humidifier water and another four patients with related complaints. The setting was a building with photographic laboratories and corresponding office rooms that were supported with air by a humidification, ventilation, and air conditioning (HVAC) system. In the humidifier water, we found different fungi, particularly species of the genus Verticillium, as well as gramnegative bacteria, but no Legionella species. Colony counts were done according to the German Drinking Water Regulations and the VDI Guideline 6022 at 20 degrees C and 36 degrees C; the results exceeded the recommended level of 103 cfu/ml. The hygienic role of the isolated microorganisms and their products (e.g., antigens, endotoxins) is discussed. The case report emphasizes the importance of careful maintenance and control of HVAC systems. The new VDI guideline 6022 (issue July 1998) gives detailed information on this subject and pays special attention to qualifications and training of the HVAC operations staff.