This prospective in vivo cross-over study investigated the effect of Intralipid on immune responses. Twenty-three patients were randomly allocated to receive one of two alternative total parenteral nutrition (TPN) regimens for the first 7 days and the other regimen for the second 7 days. Only one of the regimens included a fat emulsion to provide 50 per cent of the calorific requirement. Immunological studies were performed on days 0, 7 and 14. These included peripheral blood T cell subsets, antibody dependent cellular cytotoxicity and basal and maximal Interleukin 2 production. All immunological parameters were significantly augmented during total parenteral nutrition using the lipid based regimen. No such change was seen during intravenous feeding with carbohydrate based TPN. It is concluded that, far from being immunosuppressive, the incorporation of a fat emulsion into a TPN regimen has immunostimulatory properties.