that intravenous contrast administration (presumably because of a relative but temporary increase in capillary blood volume causing partial displacement of air) can unpredictably increase lung density. Disease processes which lead to partial filling of the air spaces, thickening of the interstitium, partial collapse of alveoli and/or an increased capillary blood volume will also manifest as a pattern of ground-glass opacification. In clinical practice, the recognised causes of ground-glass opacities on HRCT include pulmonary oedema (cardiogenic or otherwise), infections (e.g. pneumocystis jiroveci pneumonia) and some of the idiopathic interstitial pneumonias (e.g. non-specific interstitial pneumonia and respiratory bronchiolitis-associated interstitial lung disease). The presentation will review and revise the causes of physiological and disease-related ground-glass opacification on HRCT. Ground-glass opacity (GGO) is defined as increased attenuation of the lung parenchyma without obscuration of the pulmonary vascular markings on CT images. GGO may be the result of a variety of interstitial and alveolar infectious and noninfectious inflammatory diseases. As an imaging finding alone GGO does usually not allow a specific diagnosis. GGO in inflammatory disorders often will be present in the company of other interstitial or alveolar findings. However, the number of diseases that cause diffuse isolated GGO or GGO as the predominant finding is relatively small and can be prioritised with clinical information. The most common cause of diffuse isolated GGO in immunocompromised hosts are a variety of diffuse, opportunistic pneumonias, e.g. Pneumocystis jiroveci pneumonia (PCP), cytomegalovirus pneumonia (CMV) or herpes simplex pneumonia (HSV), that constitute the first differential. Chronic onset disorders in immunocompetent patients include cellular nonspecific interstitial pneumonia (NSIP), subacute hypersensitivity pneumonitis (HP), organising pneumonia, air-space sarcoid and drug-induced lung disease. In these disorders, ancillary findings such as an associated reticular pattern with traction bronchiectasis/bronchiolectasis (NSIP), mediastinal lymphadenopathy (sarcoidosis), superimposed nodularity or cysts may help to refine the diagnosis.