In some countries a diagnosis of occupational asthma for medicolegal purposes is made when a questionnaire is suggestive and the individual is exposed to a product known to be a sensitizer. The value of an open questionnaire administered by physicians with experience in occupational asthma is unknown, however. We prospectively assessed all subjects (162) referred to our clinic because their physicians thought their asthma might be work related. The medical questionnaire included questions about the nature of the symptoms (dyspnea, wheezing, cough, and chest tightness) and the timing (worse at work or after a shift at work or improved during weekends or holidays). An initial clinical assessment was set by the physician on a scale that ranged from highly probable to probable, uncertain, unlikely, or absent. Subjects then underwent objective assessment with specific inhalation challenges (n = 72), serial monitoring of peak expiratory flow rates for periods at work and away from work (n = 29), or both (n = 61), to confirm or rule out the diagnosis of occupational asthma. A total of 75 subjects (46%) were shown to have occupational asthma. Symptoms alone (type and timing) did not provide a satisfactory differentiation between those subjects with and those without occupational asthma. For example, 66 of 75 (88%) subjects with occupational asthma said that their symptoms improved during holidays, but 66 of 87 (76%) subjects without occupational asthma experienced a similar improvement.(ABSTRACT TRUNCATED AT 250 WORDS)