An appreciation of functional and morphological characteristics is fundamental to the understanding of diffuse lung disease. The detailed information available from high-resolution computed tomography (HRCT) and the facility to subjectively or objectively quantify disease has elucidated the sometimes complex pulmonary function profiles of several diffuse lung diseases. The many reasons why correlations between the extent of HRCT abnormalities and physiological measures of disease may be less strong than expected and are considered in this review.