Lung functions were measured on 13-day-old conjoined thora-copagus twins. Mean values for tidal volume and minute ventilation were 14.8 ml and 1102 ml/min for twin A and 12.8 ml and 963 ml/ min for twin B. Functional residual capacities (FRC) were 35 ml/ kg and 39 ml/kg for twins A and B, respectively. Pulmonary compliance and flow resistance for twin A were 2.8 ml/cm H2O and 75 cm H2O/liter sec−1with 45% of the total work used to overcome elastic resistance. Although values for resistance and work of breathing are within the normal range for studies reported elsewhere, these results tend to be elevated and probably reflect an abnormal state.Speculation With decreasing reluctance by surgeons to separate conjoined thoracopagus twins, data concerning their preoperative respiratory pathophysiology assumes greater importance. Pulmonary function tests on conjoined twins may provide valuable information concerning the feasibility and prognosis for successful separation.