Evaluation of medical decision support software (MDSS) -computer programs to assist health professionals with diagnostic and/or therapeutic decisions -has not kept pace with the development of such programs. This article describes the following formative evaluation issues that must be addressed by developers of MDSS to evaluate these programs properly: (1) How can systematic feedback be obtained about an evolving program? (2) How can enough data to evaluate the program be obtained? (3) How much instruction is necessary? (4) What are the most important aspects for users to evaluate? and (5) How can the appropriate use of a developing MDSS be assured? Data from an ongoing evaluation of an existing MDSS, Quick Medical Reference, are used to illustrate the issues and to suggest recommendations for addressing them. © 1989, SAGE PUBLICATIONS, INC. All rights reserved.