Patients with large primary breast cancers are being treated with neo-adjuvant chemotherapy. Studies in animals have shown that responses to chemotherapy can be increased by dietary manipulation of tumour cell metabolism. Also dietary supplementation with the amino acid L-arginine, resulted in an increase in tumour metabolic activity expression of the nuclear activation antigen, Ki67, in patients with breast cancer. Therefore, we have carried out a randomised, double blind, placebo controlled trial to determine if L-arginine supplementation is beneficial in patients with breast cancer, undergoing neo-adjuvant chemotherapy. 96 patients were randomised to receive L-arginine (30 g/day) for three days (n = 48) or placebo (n = 48) prior to undergoing chemotherapy (doxorubicin, cyclophosphamide, vincristine, prednisolone), 6 pulses at 21-day intervals. Clinical and pathological responses were assessed in both groups of patients following completion of chemotherapy. The clinical response rate was 77% (23% complete and 54% partial responses) in the L-arginine treated group, compared with 71% (15% complete and 56% partial) in the placebo group of patients (p = ns). However, in patients with tumours less than 6 cm in initial diameter, there was a significant increase in the better histopathological responses in the L-arginine group, when compared with the placebo group of patients (88% vs 52%, p = 0.04). This may have important implications for clinical practice.