A total of 230 persons who had been exposed to formaldehyde and suffered from asthma-like respiratory symptoms were examined between January 1, 1977, and May 31, 1983. All the subjects had a bronchial provocation test with formaldehyde. On the basis of the medical and occupational history of the patients, the specific bronchial provocation test, and other test results, 12 cases were considered to be caused by specific sensitization to formaldehyde. All subjects had been exposed occupationally. An exposure period of between 1 mo and 19 yr preceded the onset of symptoms. Three persons displayed no bronchial hyperreactivity as assessed with a histamine or metacholine provocation test. Eleven of the 12 reactions were triggered by about 2.5 mg/m3 and one reaction by about 1.2 mg/m3 of formaldehyde. The late reaction in 1 patient was completely blocked by the inhalation of 100 micrograms of beclomethasone di-isoproprionate before the challenge with formaldehyde. Seventy-one of the 218 subjects who did not react when they were challenged with formaldehyde demonstrated bronchial hyperreactivity. We conclude that formaldehyde asthma, although apparently a rare disease, is under reported. Removal from exposure has a favorable effect on the symptoms. Low domestic exposures, however, may maintain the symptoms in individuals already sensitized.