Costs and getting ethical approval deter doctors from participating in multicentre trials.

EDITOR,—Many questions in medicine can be answered only if large numbers of patients are studied, and multicentre trials are normally necessary for this. The research committee of the British Thoracic Society, which has conducted many multicentre trials,1 designed two studies on the prevention and treatment of osteoporosis by hormone replacement therapy and/or etidronate and/or calcium in asthmatic patients taking cor-ticosteroids. Height, weight, activity status, fractures, and (if available) results of bone densitometry were recorded annually for five years. Lateral radiographs of the thoracolumbar spine were obtained at …