Cognitive function was measured before and after inpatient treatment for metabolic control in 20 elderly patients with non-insulin-dependent diabetes mellitus (NIDDM). Another 20 patients still on the waiting list for this treatment, served as a control group. Glycosylated hemoglobin decreased in both groups. Psychomotor speed and concentration improved only after inpatient treatment (p < 0.01; p < 0.05, respectively). Improved performance was maintained and even enhanced 6 weeks after discharge from inpatient treatment. Performance in concentration tasks correlated with glycosylated hemoglobin (p < 0.05). It is concluded that cognitive deficits in elderly NIDDM patients can be reduced by inpatient treatment, although the benefit of glycemic control was not clearly demonstrated in this study.