OBJECTIVE Miglitol, an alpha-glucosidase inhibitor, delays absorption of carbohydrates. This study was undertaken to determine the potential of this agent as an adjunct to insulin in the treatment of diabetes. METHODS Twelve nonobese patients with insulin-dependent (type I) diabetes mellitus were randomly selected from the outpatient diabetes clinic. The patients were made euglycemic with the Biostator, and postprandial hyperglycemia was determined under the following conditions: protocol 1, subcutaneous injection of insulin (13 +/- 1 U) given 60 min before the meal, with insulin dosages determined by the Biostator; protocols 2 and 3 same as protocol 1 but with insulin given at the time of meal ingestion; protocols 4 and 5 same as protocol 1 but with insulin given 30 min before the meal. Miglitol (100 mg) was administered in protocols 2 and 4 and placebo in protocols 3 and 5. RESULTS When insulin was given 30 min before the meal with miglitol (protocol 4) or placebo (protocol 5), plasma glucose increased from 4.94 +/- 0.16 to 5.94 +/- 0.55 mM and from 5.11 +/- 0.22 to 8.22 +/- 0.72 mM, respectively (P less than 0.01). When insulin was given at the time of the meal with miglitol (protocol 2) or placebo (protocol 3), plasma glucose increased from 5.44 +/- 0.27 to 7.77 +/- 0.5 mM and from 5.72 +/- 0.22 to 10.83 +/- 0.77 mM, respectively (P less than 0.01). When insulin was given 60 min before the meal (protocol 1), plasma glucose initially decreased from 5.61 +/- 0.38 to 4.33 +/- 0.33 mM and then increased to 6.94 +/- 0.66 mM after the meal.(ABSTRACT TRUNCATED AT 250 WORDS)