The steady-state apparent total body clearances (TBC) of continuously infused 5-fluorouracil (5-FU) were determined in 16 adult male patients who were receiving combined 5-FU-radiation therapy. Seven patients (group A) had Stage III adenocarcinoma or epidermoid lung carcinomas; none had known metastases. Five patients had gastrointestinal carcinomas (group B), without known hepatic metastases. Four had gastrointestinal carcinomas with evidence of hepatic metastases (group C). TBCs were calculated from infusion rates and serum 5-FU concentration data. The means (standard errors) of the TBCs were: group A, 4.49 (0.53); group B, 6.51 (0.53); group C, 2.96 (0.91) L/kg/h. The difference among the means was found to be significant (p less than 0.002) by one-way ANOVA. Differences among the groups were then examined using 95 percent confidence intervals: group A was not different from group B or group C; however, groups B and C differed from each other. Patients with hepatic metastases have 5-FU TBCs about half that of those found in patients without hepatic involvement. Clinically, patients in group C attained the higher serum 5-FU concentrations known to be radiosensitizing more frequently than patients in the other two groups; differences in toxicity were not evident. This suggests that administration of an agent that can compete with 5-FU for hepatic metabolism to patients with rapid clearance might improve the therapeutic efficacy of 5-FU.