A major problem with pharmacologic treatments for cancer is the unpredictable nature of the clinical response. Therefore, many patients are treated but few benefit from chemotherapy. Selection of patients for drug treatment would greatly benefit both responders and nonresponders. Mitomycin C (MMC) and 5-fluorouracil (5-FU) are important drugs widely used in the treatment of patients with gastrointestinal malignancies. In a prospective study, an in vitro chemoresistance-sensitivity assay (CR-SA) was performed for 95 patients at the time of surgery for peritoneal carcinomatosis from colorectal and appendiceal cancer. Following cytoreductive surgery, all of these patients had minimal-to-moderate residual disease. All patients were treated with the same chemotherapy regimen regardless of the results of the in vitro assay in the postoperative period. Clinical status of patients was correlated to the assay predictions, and the results were statistically evaluated. When resistance was correlated with outcome, there was no statistical difference. In addition, the mean percentage of growth inhibition was not increased when responders and nonresponders were compared. Finally, more patients who had > or = 95% in vitro growth inhibition of cancer did not survive than did those with < or = 95% growth inhibition. The in vitro test did not predict sensitivity or resistance to cancer when regional chemotherapy was administered in a clinical setting of peritoneal carcinomatosis.