Because of current concern regarding possible adverse health effects of formaldehyde, 2 renal dialysis unit nurses shown to have formaldehyde asthma by inhalation provocation tests in 1973 and 1975, respectively, were studied. Their original tests were repeated and the formaldehyde levels generated (previously unknown) were measured. One nurse had not worked with formaldehyde since 1976 and had no further symptoms. Her 1981 test (15-min exposure to 6 ppm formaldehyde) provoked no asthmatic response. The other nurse had continued to work with formaldehyde, although under much improved conditions, and mild intermittent attacks of asthma had continued. Her test (5-min exposure to 3 ppm formaldehyde) provoked a late asthmatic reaction similar to that observed in 1975. In sensitized subjects specific late asthmatic reactions may be provoked by brief exposures to formaldehyde at about the current Occupational Safety and Health Administration [USA] standard (3 ppm); and that while asthmatic responsiveness may persist with continuing low levels of intermittent exposure, it may be lost following complete cessation of exposure.