Positron emission tomography/computed tomography (PET/CT) has become essential for oncological evaluation. In recent years, its usefulness has been shown for evaluating other non-malignant inflammatory diseases, which may present an increase in measurable metabolism. The PET/CT has the advantage of being able to detect diseases even when they do not correlate in the morphological images, also allowing to precisely locate these alterations. Among these pathologies are the study of fever of unknown origin, inflammatory diseases, connective tissue diseases, vasculitis, and also in the monitoring and diagnosis of some infectious diseases. A review of literature regarding the usefulness of PET/CT in these pathologies will be carried out in addition to clinical cases.