Eighteen patients with localized colorectal carcinoma were randomized to receive intravenous nutrition or to be fasted during the 24 h before surgery. Protein synthesis, an index of tumour growth, was then measured by the incorporation of [13C]leucine into tumour protein immediately before surgery. The mean (s.e.m.) rate of tumour protein synthesis in patients receiving nutrition (42.7(3.5) per cent per day) was 89 per cent higher than the rate in the fasted group (22.6(1.9) per cent per day) (P = 0.002). As tumours consist of a variety of different cell types, in vitro rates of protein synthesis were measured in malignant cells isolated from colorectal tumours and cultured with autologous serum obtained from the patient in either the fasted or the fed state. There was a mean increase of 81 per cent in protein synthesis when fed rather than fasted serum was used (P less than 0.02), indicating that the malignant cells themselves respond to nutrient supply. This increase in tumour protein synthesis provides the first evidence in vivo that the exogenous supply of nutrients can modulate the rate of growth of a human tumour.