Occupational asthma is best verified by physiological measures, including (1) measurement of lung function on a single occasion with bronchodilator response, (2) measurement of lung function before and after a workshift, and (3) repeated measurement of PEF over long periods of time, including readings at and away from work. The author describes a qualitative method for distinguishing patterns of PEF change at and away from work which has been effective in the diagnosis of work-related asthma.