Fifty-two instances of opportunistic lung disease were reviewed with regard to their roentgenologic presentation and course. Familiarity with the underlying disease and its therapy must be brought into focus with an understanding of the differing patterns of opportunistic lung disease both clinically and roentgenologically. Emphasis should be placed on the alveolar nature of Pneumocystis carinii pneumonia rather than the previously described interstitial, perihilar roentgenographic pattern. In its florid presentation, diffuse alveolar opacification of the lung fields is the hallmark of this disease roentgenologically. Nocardiosis should be suggested by an infiltrative or nodular lesion with cavitation, particularly in a patient having undergone transplantation and subsequent immune suppression. The aggressive tendencies of the Phycomycetes in a small group of patients were characterized by fistula formation, vascular and tracheobronchial invasion. By comparison, the benign ill-defined nodular infiltrates of cytomegalovirus infection formed a striking contrast, with a propensity for upper lobe distribution. Aspergillosis should be suggested by a "shaggy nodule" appearance which rapidly excavates. The broad group of bacterial organisms could be suggested by the nodular embolic form of the disease, more commonly seen with staphylococcal pneumonia as well as pneumonia with lobar expansion, seen with the gram negative organisms. Knowledge of the above presenting roentgenologic patterns and an awareness of the frequency of opportunistic disease should enhance our diagnostic accuracy and lead to earlier and more aggressive methods of diagnosis and treatment.