Effective intraperitoneal (IP) chemotherapy and colloidal radiation therapy depend on contact of the drugs with the entire peritoneal surface. Using technetium-99m sulfur colloid, one can evaluate the completeness of peritoneal fluid distribution. Forty-eight planar IP scans from 26 patients with gastrointestinal malignancies were reviewed and scan findings were correlated with the site of primary tumor, surgical findings, and the location of any residual disease. Of the 26 patients, 22 had undergone resection of the primary lesion prior to scanning. Scans typically demonstrated more activity in the upper and lower portions of the abdomen than in the midabdomen. Diminished uptake in the pelvis, right lower quadrant, and right upper quadrant was generally due to metastases. A tumor mass was identified as responsible in only one of eight patients with diminished or absent left upper quadrant activity. Knowledge of variability in patterns seen on IP scans is essential to rational guidance of IP chemotherapy and radiation therapy.