The direct effect and safety of carmofur (Mifurol) in cases of advanced and recurrent breast cancer were evaluated as a cooperative study at twenty-two facilities nationwide. Carmofur 12 mg/kg/day within the maximum daily dose of 600 mg was administered orally for eight weeks or longer. Of 42 patients registered for the study, 38 met the eligibility criterial (qualified patients), of whom 31 completed all tests for evaluation (completely qualified patients). The response rates were 18.4% with the qualified patients (PR, 7; Long NC, 3; NC, 11; PD, 12; NE, 5; ie, 7/38), and 22.6% with the completely qualified patients (PR, 7; Long NC, 2; NC, 10; PD, 12; ie, 7/31). The interval before the onset of PR after administration of carmofur was started was a minimum 29 days, a maximum 190 days and a median 82 days. Side effects were observed in eighteen patients (42.9%). Frequent micturition, loss of appetite, dizziness or feverish feeling were major symptoms, but none were serious.