From the early sixteenth century, uroscopy lost much of the great appeal it had possessed among medieval physicians. Once valued as an outstanding diagnostic tool which ensured authority and fame, it became an object of massive criticism if not derision. As this paper shows, growing awareness of theoretical inconsistencies, the new medical empiricism and humanistic opposition against Arabic and medieval predecessors can explain this drastic revaluation only in part. Uroscopy, it is argued here, came to be perceived above all as a threat to the physicians' professional authority. Faced with persistent demands that they diagnose diseases primarily if not exclusively from urine, they were left with an awkward choice. They risked making fools of themselves by blatant misdiagnosis, but if they rejected the patients' demands people would deem them incapable of a task which many of their less educated competitors were perfectly happy to perform. In the end, in spite of the physicians' massive campaign against it, uroscopy remained very much alive. On the highly competitive early modern medical market patient power had once more prevailed.