The SDI system now operational at the Institute for Atomic Research, Iowa State University, is implemented with the concept of feedback. The profile user is notified of a document which may be of interest to him and indicates his response to this document on the response card accompanying the notification. The profile words carry individual significance values which are adjusted according to the responses of the user. So the importance of a profile word to the user in the next dissemination run is dependent on his responses to the notifications he has previously received. The feedback functions are designed to provide stability in the feedback loop so that an equilibrium value is maintained for a large variety of profile words. The use of feedback is credited with significantly improving the results obtained by this system by creating a more accurate description of the interests of the user. A feedback function and the operational results which have been collected to date are described. (Author)