This study reports on the relationship of airway hyperresponsiveness (AH) with respiratory symptoms and diurnal peak flow expiratory (PEF) variation in 221 hyperresponsive patients with moderately severe airways obstruction. The disease was in a stable phase in all patients. Closely adhering to the American Thoracic Society criteria, patients were divided into three syndrome diagnoses based on a standardized history: asthma (n = 81), asthmatic bronchitis (AB, n = 69), and chronic obstructive pulmonary disease [( COPD] n = 44); 27 subjects could not be placed in any group. Mean (+/- SEM) log2 PC20 histamine values were significantly lower in the asthmatic group (-2.77 +/- 0.20 mg/ml) than in the COPD (-0.89 +/- 0.29 mg/ml) and AB groups (-1.37 +/- 0.25 mg/ml; one-way ANOVA, p less than 0.001). However, considerable overlap of individual responses existed. Differences between the groups could not be attributed to differences in prechallenge FEV1 levels. For every level of FEV1, asthmatic subjects were more hyperresponsive than patients with COPD. The dependence of PC20 on prechallenge FEV1 was comparable in all groups. There was a significant correlation between the degree of AH and diurnal PEF variation (rho = -0.401, p less than 0.001), which was stronger in asthma (rho = -0.409) than in COPD (rho = -0.325). Despite this obvious association, a wide range of diurnal PEF variation values existed for every level of PC20, indicating that PEF variability and AH are not interchangeable. The relationships between symptoms and both AH levels and PEF variation were weak. No significant differences were found between syndrome diagnosis groups with respect to diurnal PEF variation.(ABSTRACT TRUNCATED AT 250 WORDS)