In order to investigate the clinical significance of glucose consumption (GC) in recurrent gliomas, positron emission tomography with 18F-labeled fluorodeoxyglucose was measured in 18 cases of histologically verified recurrent gliomas. The GC of the tumors were categorized into four groups. Five tumors were in Group IV, the highest GC, four were in Group III, eight were in Group II, and one was in Group I. Masses in Groups III and IV were clearly defined as a hot spot higher than or similar to the GC of the contralateral cortex. Half of the recurrent gliomas showed the lower GC of Group I or II, but two thirds of these were histologically high-grade gliomas. Although GC in the recurrent gliomas did not always increase as expected, a focal increase of GC, even mild and small, in the area of previous surgery is diagnostically important. Tumors with high GC showed high histological malignancy, irrespective of tissue damage. Patients with tumors of low GC had longer survival rates than those with high GC, although statistical significance was not obtained. Thus, positron emission tomography with 18F-labeled fluorodeoxyglucose was useful for detecting the recurrence of gliomas and suggesting their histological malignancy and prognosis. Care should be taken because viable tumor cells could be present in areas of low GC and small recurrent masses could be missed because of the poor spatial resolution of positron emission tomography.