SummaryIn an industrial community, Linnavuori, (population 1000) over 100 people developed fever, cough, and dyspnoea 3 to 6 hours after sauna or bathing. This epidemic began in August of 1978 and ended gradually in December. The water source of the community was a small (0.4 km2) lake; in November it was found that the water was contaminated with several bacteria, fungi, and algae. Precipitating antibodies to these bacterial and fungal antigens were not found in the sera of symptomatic patients. To show that tap water, purified by the local waterworks, had provoked the symptoms, four clinically typical patients were each twice challenged. Two of the patients visited their usual public sauna, the two others took a bath at home. Two patients inhaled home tap water aerosol, one inhaled lake water vapor and one home tap water vapor in hospital. Three to six hours after seven of the eight challenges the patients showed two or more of the following symptoms and signs: chills, fever, dry cough, dyspnoea, and inspiratory crepitant râles. Six provocations increased the amount of segmented neutrophils and in each case the lung diffusion capacity decreased significantly. Although the clinical picture resembles extrinsic allergic alveolitis, it is possible that bacterial endotoxins have induced alternative pathway complement activation and release of leukocyte pyrogens.A respiratory syndrome much resembling exogenous allergic alveolitis, and caused by micro‐organism‐contaminated water from air humidifiers and ventilation systems, has become more commonly diagnosed in the 1970's. This disease is also called hypersensitivity pneumonitis or humidifier fever. Banaszak, Thiede&Fink (1970) described four office clerks whose symptoms were shown to be caused by contamination of the office ventilation system by a thermophilic actinomycete. Inhalation tests with the homologous antigen provoked similar symptoms. Finket al.(1971)