Smartphones provide opportunities to share information and may become invaluable tools for certain health care functions, such as assisting smoking cessation or monitoring medication adverse effects. Recently, several mobile health care applications have been developed and marketed directly to non-clinician consumers. This study evaluates the accuracy of four smartphone applications intended to assess photographs of skin lesions to help users decide whether the lesion is potentially dangerous. Overall, the applications performed poorly, with three incorrectly classifying 30% or more of melanomas as unconcerning. In the more accurate fourth application, images are sent to a board-certified dermatologist for evaluation, rather than using a preset algorithm. The concern is that reliance on these dermatologic applications could delay diagnoses and ultimately harm patients.