Medwise is an interactive computer assisted medical diagnosis program running on a personal computer. The knowledge base (KB) of our Medwise system is founded on a matrix structure representation of disease profiles. This study evaluates some unique features of Medwise, namely the matrix structure, the automatic assignment of weight factors during expansion of the KB, the auxiliary KB with equivalent terms, and the invariance during expansion of the KB. For 104 patients described in 1986 and 1987, the diagnostic outcome was compared to the conclusions of the clinicians. With the use of the matrix and all other options active, Medwise yields the correct diagnosis in 93% of the cases. The performance decreases to 79% if the characteristics of the matrix are eliminated. After the KB was expanded from 2,400 to 3,600 disease profiles, the correct diagnosis was established in 91% while analyzing the same cases. This implies that it may be anticipated that the score of the diagnostic program will not appreciably decrease as the KB expands towards the final goal of 8,500 disease profiles. Furthermore, the lack of standardized medical language limits the potential use of computers in medicine. Incorporation of a separate knowledge base containing equivalent expressions proved to contribute to a better score for the Medwise program.