1. The amino acid L-arginine has been shown to enhance immune mechanisms and inhibit tumour growth in experimental animals, but although many of the immunological effects of arginine have been reproduced in man there have been few studies of its effects on human tumours. In this study the effects of arginine on human breast cancers were determined by measuring tumour protein synthesis and comparing this with immunohistochemical assessments of cell proliferation. 2. Patients with breast cancer were randomized to receive either a standard diet or arginine supplementation. At the time of surgery, the rate of tumour protein synthesis was measured by the incorporation of the stable isotope [1-13C]leucine into tumour protein. Tumours were also assessed histologically and by staining for the presence of the activation antigen Ki67. 3. The median rate of tumour protein synthesis was 10%/day (range 5.5-15.8%/day) in the control patients and 25.6%/day (range 9-37%/day) in the patients receiving arginine supplements (P less than 0.005, Wilcoxon rank sum test). The rates of protein synthesis correlated with Ki67 expression within these tumours (r = 0.78, P less than 0.001). A double-staining technique confirmed that tumour cells, rather than tumour-infiltrating lymphoreticular cells, expressed Ki67. 4. This study demonstrates that, in contrast to animal studies, L-arginine stimulates human tumours in vivo. This represents the first direct evidence that a single amino acid can modulate the behaviour of a human cancer.