This paper reports the results of a study investigating the relevance criteria used by health care professionals when seeking medical images. Data was collected from 26 participants using a think-a-loud protocol and face-to-face interviews and analysed using the Straussian version of Grounded Theory (GT). Findings show that participants made use of 26 relevance criteria, although did not agree on the most important. Our findings suggest that users apply different criteria in different situations when evaluating the relevancy of medical images. In addition, we have investigated the coverage of relevance criteria to search statements from the medical track of ImageCLEF (ImageCLEFMed). Analysis indicates that some of the criteria identified by our participants could be included in new topics used for future incarnations of the medical image retrieval track. Keywords Relevance criteria, Relevance judgment, Image retrieval, Medical imaging