Nuclear medicine is an important tool in the diagnostic evaluation of patients with a variety of nonosseous infections. In the immunocompetent population labeled leukocyte imaging is the radionuclide procedure of choice, with Gallium imaging reserved for those situations in which the leukocyte study is nondiagnostic or cannot be performed. Fever of unknown origin is caused by infection in less than one-third of cases, and therefore the number of positive leukocyte studies will be relatively low. The negative leukocyte study is also useful, however, as it has been demonstrated that a negative study excludes, with a high degree of certainty, focal infection as the cause of an FUO. In the cardiovascular system, labeled leukocyte scintigraphy is very useful for diagnosing mycotic aneurysms and infected prosthetic vascular grafts, with a sensitivity of about 90%. The specificity of the study is somewhat more variable--false positive results have been described in perigraft hematomas, graft thrombosis, bleeding, and pseudoaneurysms. In the central nervous system, labeled leukocyte imaging can provide important information about the etiology of contrast enhancing brain lesions identified on computed tomography, i.e., distinguishing between neoplasm and infection. In the immunocompromised population, typified by the AIDS patient, Gallium scintigraphy is the radionuclide procedure of choice for diagnosing opportunistic diseases. In the thorax, a normal Gallium scan, in the setting of a negative chest X-ray, virtually excludes pulmonary disease. A negative Gallium scan in a patient with an abnormal chest X-ray and Kaposi's sarcoma study suggests that the patient's respiratory problems are related to Kaposi's sarcoma. Focal pulmonary parenchymal uptake is most often associated with bacterial pneumonia, although Pneumocystis carinii pneumonia can occasionally present in this fashion. Diffuse pulmonary parenchymal uptake of Gallium can be due to numerous causes, but in general, the more intense the uptake, the greater the likelihood that the patient has P. carinii pneumonia. Lymph node uptake is most often due to lymphoma or mycobacterial disease. In the abdomen, Gallium is also useful for detecting nodal disease. but is not reliable for detecting large bowel disease. Labeled leukocyte imaging should be performed when colitis is a concern. Both 18FDG PET and 201Tl SPECT imaging of the brain are useful for distinguishing between central nervous system lymphoma and toxoplasmosis in the HIV (+) patient. On both studies, lymphoma manifests as a focus of increased tracer uptake, whereas toxoplasmosis shows little or no uptake of either tracer.