The anticancer effects of sulforaphane (SFN), which is found in cruciferous vegetables, were studied on KPL-1 human breast cancer cells in vitro and in vivo. Cell proliferation in vitro was assessed by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and tumor growth and metastasis in vivo were examined in orthotopically (right thoracic mammary fat pad) transplanted KPL-1 cells in female athymic BALB/c mice. The MTT assay showed that SFN directly inhibited KPL-1 cell growth in vitro (IC50 at 48 h, 19.1 µM; IC50 at 72 h, 17.8 µM). Athymic mice received a KPL-1 cell transplant, and SFN treatment (intraperitoneal injection of 25 or 50 mg/kg SFN) was started the next day. Mice received five injections each week during the 26-day experimental period (for a total of 20 injections). Compared with the SFN-untreated controls, SFN suppressed primary tumor growth. At the termination of the experiment, the final tumor volume was 686±94 mm3 for the control group, 516±70 mm3 (75% of control value) for the 25 mg/kg SFN group and 351±55 mm3 (51% of control value) for the 50 mg/kg SFN group. The final tumor weight was 571±69 mg in the control group, 416±63 mg (73% of the control value) in the 25 mg/kg SFN group and 338±56 mg (59% of the control value) in the 50 mg/kg SFN group. SFN caused a dose-dependent decrease in the proliferation ratio and an increase in the apoptotic ratio of the primary tumor cells. SFN treatment tended to reduce regional (axillary) lymph node metastasis. Treatment with 50 mg/kg SFN significantly inhibited KPL-1 cell growth in vivo by suppressing cell proliferation and inducing apoptosis, and it tended to reduce axillary lymph node metastasis of KPL-1 human breast cancer cell xenografts in female athymic mice.