Objective: This research was carried out with 253 patients who admitted to an oncology center in Rize, Turkey to compare the pre-diagnosis and post-diagnosis dietary habits and to determine risky dietary habits of cancer patients receiving chemotherapy between March-July 2020. Material and Methods: Within the framework of literature review and expert opinions, a questionnaire consisting of 3 parts and 56 questions in total was prepared to shed light on the eating habits of the patients. The data were obtained using face to face interview technique. The Cronbach's alpha reliability coefficient for the food consumption frequency part of the questionnaire was calculated as 0.734. The data were analyzed with the Wilcoxon test, which is one of the nonparametric tests. Statistically p<0.05 was considered significant. Results: A significant portion of the patients continue their risky dietary habits by consuming some drinks too hot and some foods too cooked/fried. The frequency of consumption of all meat and meat products, cheese types, all foods belonging to the fat group, white bread and carbonated beverages such as coffee, tea and cola decreased after the diagnosis. On the other hand, consumption of egg, milk, yoghurt, ayran/kefir, whole grain bread types, vegetables and fruits increased. In this sense, many findings with high statistical significance have been reached (p<0.05). It is understood that the dietary habits of the patients have changed in many respects and they generally prefer healthy dietary habits after the diagnosis. Conclusion: To protect the society from cancer and for a healthy future programs should be prepared to increase awareness of dietary habits.