An increasing number of smartphone and software applications (“apps”) have been developed and marketed to assist in the process of diagnosis, yet little attention has been paid to their content, claims, potential risks, limitations or benefits of their use. This study sought to describe and catalogue available diagnosis apps and explore their impact on the diagnostic process. We undertook a content analysis of the app descriptions and developers’ websites using the descriptions provided for 131 medical diagnosis smartphone apps that were available in the Google Play and Apple App stores. Each app was reviewed for its content and approach, and its claims to medical authority. Four major categories of apps were identified: 1. apps for diagnosing; 2. diagnosis coding apps; 3. books, journals, or other publications in app format; 4. apps for medical education. Our analysis found that while these apps provide access to medical information previously widely not available to lay users and offered a convenient diagnostic tool for practitioners, many failed to describe the evidence base underpinning, or any other credential supporting, their design and use. These apps potentially shift how diagnosis operates, reconfiguring disease concepts and lay-professional relations. However they also raise the risk of conflict of interest and presenting inaccurate information. Further research is required into how these apps are used, the implications for medical practice and the impact on doctor-patient relationship.