We studied pulmonary function tests, maximal voluntary ventilation, arterial blood gases, and respiratory muscle strength and recruitment pattern in a 37-yr-old symptomatic man before and after surgical plication for a left unilateral diaphragmatic paralysis. After plication, FVC, FEV1, TLC and FRC increased, whereas residual volume remained unchanged. Arterial PO2 improved from 70 to 87 mm Hg. Diaphragmatic strength, as expressed by the maximal transdiaphragmatic pressure increased from 30 to 75 cm H2O, and maximal voluntary ventilation increased from 74 to 123 L/min. Ventilatory muscle recruitment also changed: there was a shift from a positive to a negative delta Pg/delta Ppl slope during tidal breathing. This indicates more effective diaphragmatic recruitment after the procedure. We conclude that surgical plication may be of benefit to patients with symptomatic unilateral diaphragmatic paralysis. The improvement is due to improved respiratory muscle function.