Fat-based total parenteral nutrition (TPN) has been shown to maintain the host nutritionally equivalent to carbohydrate-based TPN in a rat model; however, data on body composition have not been obtained. This study compared the effects of a lipid-based TPN regimen to those of an isocaloric glucose-based regimen and an oral diet on the composition of the carcass and organs of tumor- and nontumor-bearing rats. Sprague-Dawley rats implanted with the Walker 256 carcinosarcoma were randomly assigned to either diet A, a glucose-based TPN regimen; B, a lipid-based TPN regimen; or C, a purified oral diet. Tumor-bearing rats infused with diet B had less protein and more fat in their carcasses than those in the other dietary groups. Organs of nontumor- and tumor-bearing rats fed diet B contained less protein and more fat and triglycerides than rats fed either diet A or C. Survival index and hematocrit values were lowest in rats infused with the parenteral lipid diet. These findings indicate an abnormal pathological response to a TPN diet formulated to deliver 67% of nonprotein kilocalories as lipid.