The 150 consecutive patients who were hospitalized for breast cancer at our department between September 1992 and November 1995 were enrolled in this open study after having given their informed consent. Patients were assigned randomly to receive ofloxacin in daily doses of 200 mg to be taken in a single dose or a total dose of 600 mg divided into three doses postoperatively for 5 days. Ten patients (8 found to be without malignancies) were excluded from evaluation. Wound infections developed in 8 (12%) of the 69 patients in the 200-mg group and 12 (17%) of the 71 patients in the 600-mg group. Postoperative wound care lasted a median of 19 days (25th and 75th percentiles, 15 and 29 days) for patients given 200 mg and 16 days (13 and 25 days) for those given 600 mg; the difference was not statistically significant (Wilcoxon’s U test). Signs or symptoms suggesting ofloxacin toxicity were not observed, and several significant differences were found in laboratory test results between the two groups both before and after treatment. Abnormal changes after treatment were found in many individuals, but the rate of occurrence was similar in the two groups (U test). When ofloxacin is used for prophylaxis after breast surgery, we recommend a once-daily administration of 200 mg orally for financial reasons and increased patient compliance.