Changes in tumor volume after radiochemotherapy were studied in 48 patients with anaplastic glioma (glioblastoma) by means of repeat contrast-enhanced (CE) computed tomography (CT). The tumor volume was determined by calculation of the area of the CE lesion on three-dimentional CT slices. Three patterns of tumor response were identified. In the eight patients in Group 1 (17%), the tumor volume progressively increased following the completion of therapy. The tumors in the 23 patients in Group 2 (48%) showed little change for 5.6±2.7 months after therapy, and increased progressively thereafter. In Group 3, the 17 patients' (35%) tumors shrank by more than 30% after treatment. A maximum reduction of 68±17% was observed 6.4± 3.2 months post-treatment, and the tumors exhibited regrowth 11.7±4.3 months after therapy. In 11 Group 2 and six Group 3 patients, the tumor volume transiently increased by 3 to 36% 1.5±1.1 months after therapy. In 14 of these 17 patients, perifocal low density surrounded the CE lesion. The tumor expansion abated within 2 months, and was considered a transient reaction to the radiochemotherapy.