Among 154 referrals to a university hospital clinic for assessment of possible occupational asthma, the feasibility and results of different investigations were assessed using a consistent approach to all patients. A positive skin test to a workplace allergen (14 percent of all subjects), positive peak flow workplace changes (12 percent), improvement in methacholine response on holiday (9 percent), and/or positive specific challenge testing (14 percent) supported the diagnosis of occupational asthma in 61 subjects (39 percent of the total referrals). Fifty-one of these were related to a workplace sensitizer and ten to a presumed irritant. Occupational asthma was excluded in 48 subjects (31 percent) who had normal methacholine responsiveness within 24 hours of work (22 percent of the 154 subjects), peak flow readings no worse at work than on holidays (14 percent of the total referrals) and/or negative specific challenge testing (10 percent of the total referrals). Insufficient information could be obtained for a diagnosis in the remaining 45 subjects (28 percent). No single investigation was considered diagnostic in this study, as each could be positive or negative for other reasons.