To develop an accurate method to measure the volume displaced by diaphragm motion (Vdi) breath by breath, we compared Vdi measured by a previously evaluated biplanar radiographic method (Singh B, Eastwood PR, and Finucane KE. J Appl Physiol 91: 1913-1923, 2001) at several lung volumes during vital capacity inspirations in 10 healthy and nine hyperinflated subjects with 1) Vdi measured from the same chest X-rays by two previously described uniplanar methods (Petroll WM, Knight H, and Rochester DF. J Appl Physiol 69: 2175-2182, 1990; Verschakelen JA, Deschepper K, and Demendts M. J Appl Physiol 72: 1536-1540, 1992) and a proposed method that considered actual cross-sectional shape of the rib cage and spinal volume (Vdi_S); and 2) Vdi_S measured by lateral fluoroscopy in the same 10 healthy subjects. Relative to biplanar Vdi, Vdi_S values from lateral chest X-rays and fluoroscopy were not different, whereas Vdi values of Petroll et al. and Verschakelen et al. were increased by (means ± SD) 1.98 ± 1.59 and 1.16 ± 0.82 liters, respectively (both P < 0.001). During quiet breathing, Vdi_S by lateral fluoroscopy was 66 ± 16% of tidal volume and similar to that between functional residual capacity and one-half inspiratory capacity by the biplanar radiographic method. We conclude that accurate breath-by-breath measurements of Vdi can be made by using lateral fluoroscopy.