We postulated that the variation of maximal voluntary inspiratory pressures (PI,max and Pdi,max) among individuals largely reflects the variation of the structural attributes of the inspiratory muscles, in particular the muscular cross-sectional area of the diaphragm (CSAdi) and its axially projected area (A(thor)). To test this postulate, we measured PI,max in 36 healthy subjects, including 3 children and 15 weight-lifters, and Pdi,max in 11 subjects. Structural measurements by ultrasonography and anthropometric calipers were available as reported in the companion manuscript. We found a high degree of correlation of Pdi,max with diaphragm thickness (tdi), CSAdi, and CSAdi/A(thor) (r2 = 0.89, 0.89, and 0.77, respectively). PI,max was also correlated with diaphragm structural measurements, although less well. The weight-lifters had greater pressures, thicker diaphragms, and greater diaphragm maximal stress (sigma(max)) than adults of similar stature who had not trained with weights. We conclude (1) that both Pdi,max and PI,max reflect in part structural attributes of the respiratory muscles; (2) that the variation of maximal transdiaphragmatic pressures is largely attributable to the normal variation of diaphragm structure; (3) weight lifting increases diaphragm structure and pressures.