Symptoms consistent with hypersensitivity pneumonitis (HP) occurred in 26 of 50 employees working in a localized area of a large factory. This area was served by a single heating-cooling unit utilizing a water humidification system. The illness consisted of flu-like symptoms with fever, chills, headache, cough, dyspnea. Most of the subjects affected demonstrated precipitating antibodies to a variety of organisms associated with HP, and inhalation challenge with water from the humidification system resulted in the reproduction of symptoms in one employee with a history suggestive of HP. Removal of the humidification system has resulted in a "cure" in that symptoms have not recurred during a one year followup period since the removal.