Positron emission tomographic scanning with fludeoxyglucose F 18 (18F-fluorodeoxyglucose) was used to study acute changes in gliomas after chemotherapy. In six experimental subjects, scans were obtained before and at days 1, 7, and 30 after treatment. Five control patients with gliomas who did not undergo chemotherapy had two scans, 1 month apart. Ratios were calculated between peak tumor regional cerebral metabolic rate for glucose and contralateral white matter. The percent change in ratios relative to each patient's baseline scan was calculated. Ratios in three stable controls remained unchanged over the study interval; in two controls it increased 155% and 36% and both died of tumor progression. In experimental subjects, ratios increased 20% to 100% 24 hours after chemotherapy and then decreased until at 28 days they varied between 22% above and 35% below baseline. The increased fludeoxyglucose F 18 uptake at 24 hours could be from uncoupling oxidative phosphorylation or shunting glucose to ribose phosphates for salvage nucleoside synthesis.