Mean lung attenuation has been measured on computed tomography (CT) sections through lung bases of 17 patients with no evidence of respiratory disease. Sections were obtained in inspiration and at neutral respiration together with spirometric measurement of lung volume changes. The normal range for mean attenuation including all phases of respiration is approximately -350 to -430 EMI units (EU) (air = -500). The range is narrower on inspiration than at neutral respiration. Mean attenuation in the posterior one-third of the lung base may be 100 EU greater than in the anterior one-third. This gradient is reduced but not abolished on inspiration. Reduction of attenuation with inspiration correlates inversely with increased lung volume and cross-sectional area. Anteroposterior attenuation gradients and regional changes of attenuation with breathing can be explained by preferential ventilation and perfusion of the dependent regions of the lung. When using CT to measure lung "density," the effects of breathing should be taken into account. The relevance of these observations to the detection of early diffuse lung disease is discussed.