In a prospective randomized multicentre trial 139 patients with metastatic colorectal carcinoma (70 men, 69 women; age 35-81 years) were given palliative treatment with fluorouracil (400 mg/m2 daily for 5 days) alone or combined with folic acid (100 mg/m2 before each dose of fluorouracil). Both groups were comparable in respect of age, sex, Karnofsky index and number of localisations of metastases. The criterion for starting the treatment was progression of the malignancy or clinical symptoms caused by the tumour. Resulting remission rates (fluorouracil monotherapy vs combination with folic acid) were: complete or partial remission, 9 vs 16%; arrest of tumour growth, 20 vs 60%; progression 71 vs 24%. Peripheral side effects, such as stomatitis and diarrhoea, were similarly frequent with the two treatment regimens and reasonably tolerable. Median survival time for the fluorouracil monotherapy was 7.24 months from onset of treatment, and 9.1 months from the time that any metastases were diagnosed. The combination treatment with folic acid achieved a significantly longer median survival time (P less than 0.0001), 14.98 months from treatment onset and 16.3 months from metastasis diagnosis. The higher rate of response and the significantly prolonged survival time signify an improvement of the therapeutic profile of fluorouracil by addition of folic acid in the palliative therapy of colorectal carcinomas.