Background: Ketogenic diets have repeatedly been suggested for the treatment of cancer. Yet, only a few case studies have been published that reported long term benefits associated with such diets. Case report: Here we present a case where rectal cancer was treated with an animal fat-meat based diet, which we refer to as the paleolithic ketogenic diet. Upon diet initiation, the patient discontinued three medicines he had been taking because of hypertension and hyperuricemia. The patient initially received 6 weeks of radiation therapy. Thereafter the patient used the diet as a stand-alone therapy for 22.5 months. The patient was not taking any medicines or dietary supplements while on the diet. During the first five month of the diet therapy the patient exhibited excellent adherence which was paralleled by improving laboratory parameters including decreasing tumor marker levels and decreasing tumor size. The patient was in a constant ketosis during the entire follow-up. From the seventh month on, however, his adherence level was fluctuating with periods of worse adherence paralleled with negative changes in laboratory parameters. Although MRI imaging showed that there was no increase in the size of the tumor, 22 months after diagnosis onset the patient reported an alteration in bowel habits and that the bloody stool had returned. Because of the emergence of these symptoms, 24 months after diagnosis onset, rectal surgery was performed. Conclusion: With using the paleolithic ketogenic diet the patient was able to postpone surgery for two years. During the first five months, when the patient was strictly adhering to the diet, the tumor regressed. Thereafter, with incomplete dietary adherence, disease seemed to be stable but symptoms suggested progression for the last few months of the follow-up. Deviations from diet rules, even those not affecting ketosis, resulted in the progression of the disease.