Fifty-one patients with metastatic breast carcinoma were randomized to treatment with weekly 5-fluorouracil (5-FU) either by mouth or intravenously. Prior treatment included radiation therapy (75%), chemotherapy(27%), and hormonal therapy (86%). Response frequency (tumor reduction greater than or equal to 50%) in the 49 evaluable patients was 24% (6/25) for oral oral vs. 29% (7/24) for intravenous administration. Disease was stable in an additional 13% of patients treated intravenously and 20% of patients treated orally. Median survival was 9.8 months for intravenously treated patients and 12.1 months for orally treated patients (N.S.). A multivariate model of survival, to adjust for imbalance between treatment groups, also showed comparable survival in the two groups. Neither response nor survival was significantly correlated with nadir granulocyte count. Weekly oral 5-FU results in comparable response and survival to 5-FU given intravenously, consistent with a relatively low dose-response relationship for 5-FU in breast carcinoma.