Previous report suggested that weekly 24-hour infusion of high-dose 5-fluorouracil (5-FU) and leucovorin is a highly active and relatively low toxic regimen for the treatment of colorectal carcinoma (J Clin Oncol 9: 625-30, 1991). This phase II study was conducted to test this important observation by a slightly modified regimen in a larger group of patients. The weekly HDFL regimen consisted of 5-FU 2600 mg/m2/week and leucovorin 300 mg/m2/week (maximum 500 mg) in a 24-hour intravenous infusion. Between February 1992 and December 1995, a total of 42 patients with non-resectable, recurrent or metastatic colorectal adenocarcinoma were enrolled onto the study. Twenty-nine (69.0%) patients had prior exposure to lower-dose 5-FU. There were 22 men and 20 women with median age of 60 (20-75) years. They received a total of 855 and an average of 20.4 (4 to 65) courses of HDFL chemotherapy. Most patients were treated at outpatient clinics and the drugs were infused by an ambulatory pump system via a Port-A catheter. The median duration of follow-up was 22 months. ECOG Gr 2-3 stomatitis, diarrhea, nausea, and vomiting developed in 6 (14.3%), 6 (14.3%), 5 (11.9%), and 5 (11.9%) patients, respectively. Twenty (47.6%) patients had developed hand-foot syndrome. There was no hematological toxicities except 3 (7.1%) patients developed ECOG Gr 1-2 leucopenia. The overall response rate was 42.9% (28%-59%, 95% C.I.) with 2 complete responses and 16 partial responses. Eight (61.5%; 31%-86%, 95% C.I.) of 13 patients, who had no previous 5-FU exposure, responded (1 complete response, 7 partial responses). Ten (34.5%, 17%-54%, 95% C.I.) of 29 patients, who had had previous lower-dose 5-FU exposure, responded (1 complete response and 9 partial responses). The median duration of response was 5 months (1+ to 23+ months). The median overall survival of the whole group of 42 patients and the 18 responders was 10 and 22 months, respectively. Our data supported the original results of HDFL regimen in the treatment of colorectal cancers. HDFL regimen can be used either as first-line or second-line treatment for non-resectable, recurrent or metastatic colorectal cancers.