Two interactive computer-based systems have been evaluated: a teaching program with text and animated graphics and a multiple-choice knowledge-assessment program (KAP) with optional prescriptive feedback. One hundred seventy-four routine-attending insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) patients were allocated to active and control groups to determine the effect of these programs on knowledge and control after a 4- to 6-mo follow-up period. Interactive computer teaching (ICT) resulted in a significant knowledge increment in both IDDM and NIDDM patients (P < .05), together with a mean fall of 0.8 and 0.7%, respectively, in HbA1c (P < .05 and P > .1), but no changes were observed in respective control groups. The KAP with feedback also produced a significant knowledge increment in both IDDM and NIDDM patients (P < .05), of similar magnitude to the ICT program, and a mean fall in HbA1c of 1.2 and 1.3%, respectively (P < .05), with no changes in the corresponding control groups. Even when KAP was used without prescriptive feedback, smaller but significant mean falls in HbA1c of 0.7 and 0.8% (P < .05) were seen in IDDM and NIDDM patients, respectively, suggesting a motivational effect resulting from program participation. An inverse correlation was demonstrated between knowledge increment and final HbA1c in the ICT and KAP/feedback groups, which was significant for NIDDM patients (r = −.43, r = −.45; P < .05) but not for IDDM patients (r = − .2, r = .25;P=.1). Our studies confirm positive educational benefit of both computer-based programs, probably acting through enhancement of both knowledge and motivation.