The effect of body position (right and left lateral position respectively) on gas exchange was observed systematically in 15 patients with unilateral tuberculous pleural effusion. The arterial blood samples were obtained then PaO2 were measured, while the patient lay in the lateral position with normal side down (PaO2-N) for 15 min and with effusion side down (PaO2-E) for 15 min, respectively. The result showed the mean of PaO2-N is significantly higher than that of PaO2-E (P < 0.05). It was postulated that change of PaO2 related to body positions may be attributed to difference in V/Q ratio. Our results may help nurses in managing such patients.