Semmelweis’s investigations of puerperal fever are some of the most interesting in the history of medicine. This paper considers Hempel’s analysis of the Semmelweis case. It argues that this analysis is inadequate and needs to be supplemented by some Kuhnian ideas. Kuhn’s notion of paradigm needs to be modified to apply to medicine in order to take account of the classification schemes involved in medical theorising. However with a suitable modification it provides an explanation of Semmelweis’s failure which is argued (...) to be superior to some of the external reasons often given. Despite this success in applying Kuhn’s ideas to medicine, it is argued that these ideas must be further modified to take account of the fact that medicine is not a natural science but primarily a practice designed to prevent and cure diseases. (shrink)