To characterize the in vivo force-length relation of the human diaphragm, we related pressures during static inspiratory efforts (Pmus and Pdi, respiratory muscle and transdiaphragmatic pressures, respectively) to diaphragm lengths measured on chest X rays from 22 normal subjects. At total lung capacity, the intersection of diaphragm and chest wall contours corresponds to the anatomic junction of diaphragm and chest wall. This point is located by skeletal landmarks to reveal the entire diaphragm contour on films taken at lower lung volumes. To validate the X-ray measurements, corresponding diameters were measured on 32 normal diaphragms at necropsy. After correction for height and diaphragm position, in vivo and necropsy length estimates along the coronal section agreed within 9%. The diaphragm length-lung volume relation is curvilinear, with length increasing primarily in the portion of the diaphragm apposed to the chest wall. As length increases, Pmus and Pdi rise sharply then plateau, generally conforming to force-length behavior of isolated muscle. However, absence of a Pdi peak at presumed diaphragm resting length suggests that Pdi is submaximal during voluntary inspiratory effort.