The effects of glucosidase inhibition on postprandial glucose tolerance was studied in 11 insulin-dependent diabetics. In comparison with placebo, 50 mg miglitol was able to lower the incremental glucose response significantly at 30 minutes and 60 minutes when insulin was injected: (i) 30 minutes before the meal (2,3 +/- 0,5 mmol/l v. 0,37 +/- 0,2 mmol/l; P less than 0,001; and 5,0 +/- 0,7 mmol/l v. 1,1 +/- 0,8 mmol/l; P less than 0,001); and (ii) immediately before the meal (2,3 +/- 0,5 mmol/l v. 2,2 +/- 0,9 mmol/l; P less than 0,001) respectively. The incremental glucose area under the curve when insulin was injected 30 minutes before breakfast was also significantly reduced on miglitol in comparison with placebo (0,67 +/- 0,15 mmol/l v. 0,16 +/- 0,14 mmol/l; P less than 0,01). The effect of miglitol was more evident when insulin was injected 30 minutes before rather than immediately before the meal. No significant adverse effects were encountered. It is concluded that: (i) miglitol safely reduces the early post-meal glucose increments in insulin-dependent diabetics; and (ii) its effect enhances the hypoglycaemic response of an appropriately timed injection of insulin.