To evaluate Gd-DTPA contrast enhancement of brain tumors over time and to describe the dispersion of contrast into the zone of peritumoral edema. We performed MR imaging with a dose of 0.4 mmol Gd-DTPA/kg on eleven patients diagnosed with 5 different supratentorial tumors. MR imaging was done at five intervals between 5 min and 6 h. The change in zone of enhancement was measured for each time point, and a linear measurement was made of the furthest dispersion of contrast from the original volume of enhancement. An increase in the zone of enhancement over time was seen for all tumors; the average increase in volume of contrast was 14.76 +/- 3.35 cm(3) (mean +/- standard deviation). The largest changes in the zone of contrast enhancement, 18.6 +/- 4.63 cm(3), were seen in glioblastoma multiforme. The expansion of contrast enhancement assumed the morphology of the surrounding edema. The dispersion of Gd-DTPA over time into the zone of peritumoral edema is a potential source of error in clinical settings when there is a delay between Gd-DTPA injection and scanning.