SUMMARYShortness of breath is a common symptom in thyrotoxicosis and it may have a number of causes. We have studied dyspnoea, skeletal muscle power and respiratory muscle power in eleven patients who had thyrotoxicosis with no evidence of heart failure.Four patients (36·4%) had a marked improvement with treatment in the maximal inspiratory pressure developed at the mouth. All four were breathless and had a proximal myopathy before treatment. This confirms the existence of a group of thyrotoxic patients with a reversible respiratory muscle myopathy which may explain the frequent finding of breathlessness on exertion in such patients