Although computed tomography (CT) has not been used as often in air-space diseases as in interstitial ones, it often provides useful information. Diseases such as early opportunistic pneumonia may be detected by CT before becoming visible on plain radiographs. Also, extent and location are often better defined by CT than by plain radiographs. Accompanying abnormalities such as abscess, lymphadenopathy, and pleural effusion are often clearly demonstrated by CT. This article reviews the CT findings in a variety of air-space diseases, including pneumonia and other infections, radiation pneumonitis and fibrosis, embolism, eosinophilic pneumonia, alveolar proteinosis, bronchioloalveolar carcinoma, lipoid pneumonitis, sarcoidosis, and trauma.