We describe an uncommon case of hemidiaphragm dysfunction in a man with megacolon. The patient complained of exertional dyspnoea associated with a restrictive ventilatory impairment, a reduction in respiratory muscle force, and a left hemidiaphragm elevation on roentgenogram. During maximal inspiratory manoeuvre, the electromyograph (EMG) showed an abnormal pattern in the time of recruitment of the left hemidiaphragm, associated with a rapid decline in force. After abdominal surgery (total colectomy) a significant improvement in lung volumes was observed, together with a resolution of dyspnoea. Respiratory muscle force returned to normal values with a normal EMG, whilst the elevation of the left hemidiaphragm at radiography was still present, though to a lesser extent.