Leiden University boasted one of the most popular and influential medical schools of the mid-seventeenth century, drawing hundreds of students from across Europe. These students participated in the revival of frequent clinical instruction, anatomical and chymical experiments, and even tests of supposed disease-causing substances and remedies on living animals and humans. Comparing records of cases from the hospital clinic with the professors' treatises and student-authored disputations shows that old and new theories of disease and drug action were hotly contested and often tested, including the claims of the leading professors at the school. Though notable exemplars of their work received sharp criticism and rejection from contemporaries and subsequent generations, Leiden students and professors united chymistry, postmortem autopsy, anatomical experiments, and clinical tests, often aiming at discovery. They enacted one, perhaps ephemeral, instance of experimental, clinical medicine well before its putative modern birth.