This study is a comparison of the relative effectiveness of similarity-based search features from multiple online health information portals. Similarity searching is the practice of retrieving potentially related information online by identifying a known relevant datum and taking advantage of a utility whose function is to find “related” online information. In the case of online health information resources, use of these applications produces varied results. Online literature retrieval services offer users the ability to identify articles in the biomedical literature that are deemed to be similar or related in some way. Among such portals, there appears to be considerable variability in the degree of precision and recall among documents retrieved using this technique. Retrieval of related articles from Medline through various interfaces produced non-identical lists of articles. This research describes the systematic comparison of similarity functions offered by multiple online health information resources and calculates precision, recall and overlap in order to identify the most effective similarity-searching application using ANOVA analysis.