Background: In recent years, the Internet has become an important source of information for cancer patients. Various cancer diets that are publicized on the Web promise significant benefits. The aim of our study was to evaluate the quality of online patient information about cancer diets. Materials and Methods: A patient's search for ‘cancer diets' on German websites was simulated using the search engine Google. The websites were evaluated utilizing a standardized instrument with formal and content aspects. Results: An analysis of 60 websites revealed that websites from nonprofit associations as well as self-help groups offer the best content and formal ranking. Websites whose owners aim to make a profit, practices that offer cancer diet therapies, and newspapers received the poorest quality score. The majority of content provided on the Web gets published by profit-oriented content groups. Conclusion: The divergence between profit-driven websites offering low-quality content and the few trustworthy websites on cancer diets is enormous. The information given online about cancer diets may turn out to be a hazardous pitfall. In order to present evidence-based information about cancer diets, online information should be replenished to create a more accurate picture and give higher visibility to the right information.