The authors present a sigmoid mathematical model of the pressure-volume curve of the human lung, based on a relationship between the specific compliance and the maximal pulmonary volume: (see article). This model fits better the experimental data obtained in 20 young normal adults than the currently used exponential model of Salazar and Knowles (1964). The independence between the constant K' and body height is interpreted as a constancy of pulmonary structure for normal subjects of the same age range.