An interactive method for performing a differential medical diagnosis utilizing a programmed computer system and a stored data base. In one particular embodiment, the method is for making a dermatopathological diagnosis based on operator selections from data files sequentially presented from the broadest scope to the narrowest scope. The data base is stored in a forwardly and backwardly linked hierarchical array of data files. If the data base were graphically arranged according to the linkages, it would appear in the form of a decision tree having a plurality of levels, each level encompassing the entire subject matter of dermatopathology. Each data file in the data base corresponds to a node of the decision tree and the records in the data file correspond to the branches.