The impact of high-lipid intravenous nutrition (IVN) on selected indices of nitrogen retention following major surgical resection was studied. Twenty-two patients, randomly allocated to two equal well matched groups, received either high-lipid IVN (75 per cent non-protein calories supplied as lipid) or isocaloric isonitrogenous glucose IVN (100 per cent non-protein calories supplied as glucose). Total body nitrogen (assessed by in vivo neutron activation analysis), nitrogen balance and levels of circulating proteins were measured. Mean(s.d.) total body nitrogen and fat-free mass decreased (P = 0.04) in patients receiving high-lipid IVN, -109(36) gN and -1.7(0.4) kg respectively, but not in those given glucose-only IVN, 8(43) gN and 0.1(1.0) kg. This small loss of body protein does not appear to be clinically significant because postoperative hospital stay, complication rates and the acute-phase protein response (immunological and visceral) were similar in the two groups.