Present therapy for cancer patients is based primarily on tumor histology and anatomy. Despite treatment advances, clinicians have often been frustrated by their inability to assess tumor biology. Metabolic imaging by positron emission tomography is a promising new technology that has opened up a whole realm of study in oncology. Glucose metabolism, hemodynamics, oxygen utilization, protein synthesis and thymidine incorporation may yield valuable data regarding tumor aggressiveness and stage, and allow assessment of tumor response early during treatment, before morphological changes are detectable. Other metabolic determinations, including tumor hypoxia and estrogen receptor density, may predict tumor response to therapy, and lead to selection of more appropriate treatment regimens, such as with neutrons or hypoxic cell sensitizers for documentably hypoxic tumors. While further clinical validation is required, positron emission tomography promises to provide vital information complementary to present anatomical imaging modalities that will aid oncologists in optimal management of their patients.