No standard regimen has been identified for patients with a carcinoma of unknown primary (CUP). This study compared protracted venous infusion 5-fluorouracil (PVI 5-FU) with or without mitomycin C (MMC) in patients with CUP in a multicentre, prospectively randomised study. 88 patients were randomised to PVI 5-FU (300 mg/m(2)/day for a maximum of 24 weeks) +/-MMC (7 mg/m(2) 6 weekly for four courses). The overall response rate was 11.6% for PVI 5-FU alone compared with 20.0% for PVI 5-FU plus MMC (P=0.29). Median failure-free survival (FFS) was 4.1 months for PVI 5-FU and 3.6 months for PVI 5-FU plus MMC (P=0.78) with an equivalent overall survival (OS) (6.6 versus 4.7 months, P=0.60). Symptomatic benefit was observed in most patients in each arm. PVI 5-FU is a well tolerated outpatient treatment regimen for patients with CUP, although the addition of MMC provides little extra benefit. PVI 5-FU may be a potential reference regimen in randomised trials with newer chemotherapy agents in patients with CUP.