The ratio of the muscular cross-sectional area of the diaphragm (CSA(di)) to the axially projected area of the thorax (A(thor)) theoretically determines the strength of the inspiratory pump. We studied these dimensions in 37 healthy subjects by ultrasonography and anthropometry. In 21 subjects who did not train with weights, thickness of the diaphragm (t(di)), circumference of the rib cage (c(di)), and CSA(di) increased with height and with body weight. The increase of thoracic cavity dimensions with weight was similar to that described across a wide range of mammals and was consistent with the scaling principle of elastic similarity. CSA(di)/A(thor) showed considerable variability and was not systematically dependent on height or weight. The 15 adults who trained with weight-lifting had thicker diaphragms for comparable height and greater CSA(di)/A(thor) than the adults who did not train. We conclude that (1) the structural dimensions of the diaphragm and thorax show substantial variability, some of which is systematic with stature; (2) the variations of structure predict substantial variation of inspiratory strength which is not systematic with stature; (3) the muscular cross-section of the diaphragm is increased by general or specific training.