We have studied a defined group of 12 weight-losing patients with metastatic colorectal cancer to evaluate the occurrence of and possible relationship between those determinants of carbohydrate metabolism which have been reported to occur commonly in cancer cachexia. The rates of endogenous glucose production and recycling via lactate (Cori cycle) were measured following an infusion of 50 to 100 microCi of [1-14C]glucose. Compared to an age-related group of control subjects without cancer, significantly elevated rates of glucose production [136.4 +/- 9.0 (S.E.) versus 101.0 +/- 4.6 mg/kg/hr; p less than 0.01] and recycling (43.0 +/- 7.2 versus 15.4 mg/kg/hr; p less than 0.01) were observed. Values for glucose production and recycling ranged from normal to markedly elevated. Glucose tolerance was then determined following a p.o. glucose load of 40 g/sq m in 10 of the 12 patients. Compared to control subjects, all showed a significantly delayed clearance of glucose (p less than 0.01) and a blunted insulin-secretory responsiveness (p less than 0.025). Increased glucose production and recycling was only observed in the presence of carbohydrate intolerance, but the latter occurred in a manner which seemed independent of the rate of glucose turnover. In order to obtain an estimate of hepatic glycogen reserves, glucagon, 15 ng/kg/min, was infused over 40 min in seven subjects. A significantly blunted glycemic response was observed in the cancer patients compared to controls (delta 25.0 +/- 6.9 versus 57.8 +/- 8.5 mg/dl; p less than 0.025). Neither the rate of glucose production nor the glycemic response to glucagon appeared to correlate with the immediate antecedent caloric intake. An apparent relationship was observed, however, between increased glucose production and recycling and a lack of response to infused glucagon, probably reflecting decreased glycogen stores in the face of an increased glucose requirement by the patient. We have shown that diverse abnormalities of carbohydrate metabolism commonly occur in cancer cachexia and that significant metabolic heterogeneity may be expected, despite a uniform diagnosis. These results should prove useful in the interpretation and development of clinical studies on cancer cachexia.