Positron emission tomography (PET) can be used for the noninvasive biochemical analysis of lesions whose benign or malignant nature cannot be assessed by morphological methods alone. This information and the possibility of quantitation are the reasons for the successful application of PET in staging, diagnosis of tumor recurrence and therapy monitoring in different malignant tumors. Since malignant tumors show characteristic biochemical properties with an enhancement of glucose metabolism, 18fluorodeoxyglucose (FDG) has proven useful for oncologic studies. This paper deals with the influence of PET on the surgical and other therapeutic procedures in patients with cancer of the lung, thyroid, intestine, breast and soft tissue. PET is useful for the mediastinal staging of patients with lung cancer and the evaluation of persistent tissue or tumor recurrence after treatment of lung cancer and colorectal cancer. Furthermore, important information is obtained for the staging of colorectal cancer and mammary carcinoma. PET is also indicated in patients with colorectal cancer or mammary carcinoma and a rising tumor marker. An important future application is therapy monitoring. New treatment modalities will raise new problems in terms of functional diagnosis for the different imaging procedures. Therefore, future work has to concentrate on the establishment of diagnostic algorithms for the optimization of therapy.