OBJECTIVE - To compare the different therapeutic principles of a-glucosidase inhibitors and sulphonylureas as first-line treatment in non-insulin-dependent diabetes mellitus (NIDDM) patients with dietary failure. RESEARCH DESIGN AND METHODS - Ninety-six NIDDM patients (35-70 years of age, body mass index [BMI] less than or equal to 35), insufficiently treated with diet alone (HbA(1c) 7-9%) were randomized into three groups and treated for 24 weeks with acarbose, glibenclamide, or placebo. Efficacy, based on fasting blood glucose (BG), BG 1 h after ingestion of standard breakfast (postprandial), serum insulin, postprandial insulin increase, and HbA(1c); and tolerability, based on subjective symptoms and laboratory values, were investigated every 6 weeks. Efficacy evaluation was valid for 85 patients. RESULTS - The test drugs were dosed as follows: 100 mg acarbose (A) three times a day, 1 placebo tablet three times a day, 3.5 mg glibenclamide tablets dosed 1-0-0 or 1-0-1; mean dose 4.3 mg/day. Compared with the placebo, both drugs showed the same mean efficacy on fasting BG (-1.4 mM. with acarbose, -1.6 mM with glibenclamide), 1-h postprandial BG (-2.2 mM with acarbose, -1.9 mM with glibenclamide), and HbA(1c) (-1.1% with acarbose, -0.9% with glibenclamide); but they showed a marked difference in l-h postprandial insulin values (-80.7 pM with acarbose, 96.7 pM with glibenclamide). The mean relative insulin increase (1-h postprandial) was 1.5 in the placebo group, 1.1 in the acarbose group, and 2.5 in the glibenclamide group. No changes in body weight could be observed. No adverse events were seen under placebo. Acarbose led to mild or moderate intestinal symptoms in 38% of patients. Glibenclamide led to hypoglycemia, which could be solved by dose reduction, in 6% of patients. No dropouts occurred in any of the treatment groups. CONCLUSIONS - Acarbose and glibenclamide are effective drugs for the monotherapy of NIDDM patients when diet alone fails. Because postprandial insulin increase has been shown to be associated with increased risk for cardiovascular disease, acarbose, which lowers pp increase, may be superior to glibenclamide, which elevates postprandial insulin increase.