Abstract: A method and apparatus for non-invasive assessment of lung inhomogeneity by accurate high temporal resolution measurement of respiratory gas flows at the mouth during steady state breathing and inert gas wash-in or wash-out and using these measurements to fit a mathematical model of the inhomogeneous lung. The model of the lung is based on modelling the lung as plural alveolar compartments each having an identical volume at functional residual capacity, but varying in its fractional share of total lung compliance, total pulmonary vascular conductance and total anatomical deadspace. A bivariate log-normal distribution of the lung compliance and pulmonary vascular conductance is used, together with a normal distribution of deadspace fraction.