The main function of the respiratory system is gas exchange. Since many disease or injury conditions can cause biomechanical or material property changes that can alter lung function, there is a great interest in measuring regional lung ventilation. We describe a registration-based technique for estimating local lung expansion from multiple respiratory-gated CT images of the thorax. The degree of regional lung expansion is measured using the Jacobian of the registration displacement field. We compare lung expansion estimated across five pressure changes to a xenon CT based measure of specific ventilation, and have shown good agreement (linear regression, r2 = 0.89 during gas wash-in) in one animal.