The Walker 256 carcinosarcoma was shown to lack the enzyme 3-ketoacid CoA transferase. This suggests that ketone bodies cannot be used as a major substrate for the energy metabolism of this tumour. Systemic ketosis (1-2 mM acetoacetate plus 3-hydroxybutyrate) was induced both in tumour-bearing and in non-tumour-bearing rats with a diet containing 70% medium chain triglyceride. However, in rats bearing the Walker 256 tumour, this dietary ketosis did not reduce the tumour growth rate nor did it prevent the subsequent decrease in host body weight. Host body nitrogen losses were similarly unaffected. The ketosis induced in tumour bearing rats was shown to be abnormal since the blood glucose concentration of ketotic, tumour-bearing rats was significantly higher compared with that of ketotic non-tumour bearing rats (5.2 +/- 0.4 mM cf 3.4 +/- 0.6 mM, P less than 0.01). These results may partly explain why systemic ketosis failed to alter the growth and cachexia induced by the Walker 256 carcinosarcoma.