A diagnostic system and method. In one embodiment, the invention is a diagnostic system comprising: a form comprising one or more diagnostic questions and one or more possible answers to each diagnostic question; a first overlay comprising one or more windows and one or more diagnostic fields, wherein when the first overlay is positioned over the form, the answers to the diagnostic questions that are indicative of the user having a first medical condition are visible through the windows of the first overlay; and wherein the one or more diagnostic fields contain criteria for diagnosing the user with the first medical condition based on the user's selection of answers on the form that are visible through the windows of the first overlay. In other embodiments, the invention is a computer-based version of the diagnostic system and a diagnostic method.