Three-dimensional reconstruction of human diaphragm provide useful information on the functional anatomy of the respiratory system by the analysis of its geometry. The aim of the current work is the development of a new method for the 3D analysis of diaphragm geometry by the combination of free-hand Ultrasound (US) scans with an optoelectronic system of movement analysis for the tracking of the probe. 6 healthy subjects (age=24+/-2), have been measured with a free-hand US scanning of the abdomen in supine (SP) and seated position (ST), during breath-holding at Functional Residual Capacity (FRC) and Total Lung Capacity (TLC). For one of them, measurements have been repeated in supine position in an MR scan for the validation of the method. Posture has different implications on diaphragm geometry depending on the respiratory volume. At FRC the ray of curvature (rho) in ST is 101.1+/-43.1 mm higher than in SP (p=0.006), while at TLC, posture influences the position of the diaphragm with a caudo-cranial displacement from SP to ST of 23.4+/-16.7mm (p=0.019). Diaphragm geometry at different lung volumes is influenced by the posture. In SP, rho increases of 105.9+/-48.3 mm (p=0.008) and there is a cranio-caudal displacement (Deltay) of 47.54+/-15 mm (p=0.002) shifting from FRC to TLC. In ST, Deltay=31.1+/-13.5 mm (p=0.006) while rho increases not significantly. Percentage errors between MR and US 3D reconstructions are 2.1% and 10.46% for Deltay and TLC/FRC rho ratio, respectively. US 3D reconstruction is a reliable method for the assessment of diaphragm functional anatomy. Posture directly influence diaphragm geometry and hence respiratory mechanics.