The diagnosis of red cedar asthma is usually confirmed by a specific challenge with plicatic acid, the compound responsible for the disease. We performed this study to determine the sensitivity and specificity of two other diagnostic tests, prolonged recording expiratory flow rate (PEFR) and measurement of bronchial responsiveness (provocative dose of methacholine causing a 20% fall in FEV1 [PC20 methacholine]). Twenty-three patients with suspected cedar asthma participated in the study. The patients recorded PEFR during 2 weeks away from work and 3 weeks at work. PC20 was measured both at the end of the nonworking and working period. An obvious decrease in PEFR in 2 of 3 working weeks, when PEFRs of weekends or holidays were compared (by visual inspection of the PEFR recording), and a decrease in PC20 by more than a twofold dilution, when the patient returned to work, were considered as positive tests for cedar asthma. Plicatic acid challenge test was performed at the end of the study; 14 patients reacted, whereas nine patients did not. With the results of the plicatic acid challenge test as the gold standard, the sensitivity and specificity of PEFR recordings were 86% and 89%; changes in PC20, 62% and 78%; and 93% and 45% for a positive clinical history. The combination of PEFR and clinical history revealed a 100% sensitivity with a 45% specificity. Combination of PEFR and PC20 did not improve the diagnostic accuracy.(ABSTRACT TRUNCATED AT 250 WORDS)