Online health information should meet the reading level for the general public (set at sixth-grade level). Readability is a key requirement for information to be helpful and improve quality of care. The authors conducted a systematic review to evaluate the readability of online health information in the United States and Canada. Out of 3743 references, the authors included 157 cross-sectional studies evaluating 7891 websites using 13 readability scales. The mean readability grade level across websites ranged from grade 10 to 15 based on the different scales. Stratification by specialty, health condition, and type of organization producing information revealed the same findings. In conclusion, online health information in the United States and Canada has a readability level that is inappropriate for general public use. Poor readability can lead to misinformation and may have a detrimental effect on health. Efforts are needed to improve readability and the content of online health information.