Nineteen asthmatic volunteers underwent methacholine-induced asthma to the point when tightness in the chest was just sensed (threshold symptom). Changes in the following indices of lung function were measured--static lung volumes, forced expiratory volume in one second, and airways conductance. The increase in airways resistance necessary for threshold detection was at least double that previously reported in experiments using external resistive loads. Despite the mildness of the symptom, increases in lung volumes to the levels previously described during acute and severe asthma were occasionally found. It is emphasised that there may be little leeway in respiratory reserve between the development of minor and severe symptoms in some asthmatic patients.