Serum concentrations of 5-fluorouracil (5-FU) given by continuous infusion to 19 patients with advanced colonic cancer were measured by an HPLC method, and steady-state concentration (SSc), area under the curve (AUC72) and total body clearance (Cl) were calculated as pharmacokinetic parameters. The serum level of 5-FU rapidly increased, reaching a plateau within 2 h after the start of administration. There were positive correlations between the dose and both SSc (r = 0.578, P less than 0.01) and AUC72 (r = 0.558, P less than 0.05). When the patients were divided into toxic and non-toxic groups according to the degree of toxicity, the values for SSc and AUC72 in the toxic group were significantly higher than those in non-toxic patients. The Cl value in the toxic group was also significantly different from that in the non-toxic group when data were calculated on a log scale. Furthermore, no differences in these parameters between effective and non-effective in these parameters between effective and non-effective groups were detected when the patients were divided into two groups according to anti-neoplastic responses. These results indicate that increased serum concentration does not always provide therapeutic benefits to patients receiving continuous infusions of 5-FU.