Cancer cachexia is a syndrome of unintentional weight loss that is characterized by wasting of both skeletal muscle and adipose tissue. Glucose intolerance and insulin resistance have been associated with cancer cachexia. However, it is unknown whether resistance to insulin has a role in the development of cachexia. In the present study, male CD2F1 mice with colon-26 adenocarcinoma tumors underwent an insulin tolerance test before the onset of weight loss. Compared to mice without tumors, mice with tumors had a profoundly blunted blood glucose response to insulin. Corroborating these findings, mice with tumors had decreased phosphorylation of Akt in quadriceps muscle and epididymal adipose tissue at the end of the study. Expression of Akt-regulated genes Atrogin-1, MuRF-1, and Bnip3 was increased in muscle, suggesting a role for decreased insulin signaling in the induction of both proteasomal proteolysis and autophagy in cachectic muscle. Rosiglitazone treatment increased serum adiponectin, insulin sensitivity, and body weight, and decreased Atrogin-1 and MuRF-1 expression in the skeletal muscle of tumor-bearing mice. In conclusion, insulin resistance is an early event in mice with cachexia induced by colon-26 tumors. Rosiglitazone improves insulin sensitivity and decreases early markers of cachexia. These data provide evidence that insulin resistance is not only present in cachexia, but also has a role in cachexia pathogenesis. Correction of insulin resistance may be a novel therapeutic target for the treatment of cancer cachexia.