The examination of urine with the naked eye remained a central diagnostic procedure even in learned medicine until far into the eighteenth century. Indeed, it continued to be appreciated by most doctors as a highly valuable way of gaining essential insights into the nature and causes of illness. Among the general public and among parts of the medical profession it did not lose significance until well into the nineteenth century. Such a tenacious belief may seem puzzling. Why would highly educated physicians with years of scientific and philosophical training have believed that a visual examination of urine held the key to insights into the mysterious processes of disease inside the body, and that it pointed the way to an effective treatment? And what fed the unwaning belief of patients and families in this method at a time when doctors began to grow more skeptical? In order to answer these puzzling questions, we first need to take a closer look at what uroscopists actually did, what they looked out for, and how they underpinned their diagnostic reasoning. In a second step, we will examine how uroscopy worked in ordinary medical practice and how it seemed to prove its worth over and again in various ways.