Acarbose is a newly developed inhibitor of intestinal alpha-glucosidase, and in the current study its ability to lower plasma glucose levels was studied in 12 patients with non-insulin-dependent diabetes mellitus, poorly controlled on diet plus sulfonylurea drugs. Patients were studied before and three months after the addition of acarbose to their treatment program, and there was a notable fall in postprandial plasma glucose concentrations that approximated 60 mg/dL. When acarbose therapy was discontinued in five patients, plasma glucose levels rapidly returned toward pretreatment levels. In addition to the improvement in glycemia, acarbose treatment also led to a notable reduction in Hb A1c and triglyceride concentrations. Finally, considerable individual variation was noted in the response to acarbose, and the results in four patients were quite dramatic, with striking reductions in both fasting and postprandial glucose concentrations. These data suggest that acarbose may be a useful addition in the treatment of patients with non-insulin-dependent diabetes mellitus.