Cachexia and chronic inflammation are major challenges for cancer patients, leading to serious consequences. Accordingly, it is of high clinical relevance to identify early risk factors for optimal treatment, as these are currently not available. The present study demonstrates a strong decline in contact hypersensitivity, a parameter for cell-mediated immunity, in tumor-bearing cachectic mice. Interestingly, a significant reduction was already observed during the pre-cachectic state, reflecting an impaired immune function prior to weight loss. Extrapolating to the human setting, reduced immune competence of cancer patients could serve as an early marker for cancer cachexia, enabling an early supportive care strategy.