Proinsulin release is increased relative to insulin secretion in subjects with type 2 diabetes, indicative of islet dysfunction. However, it has not been conclusively shown whether there is an increased relative proinsulin release in subjects with impaired glucose tolerance (IGT), i.e. whether it precedes the development of diabetes. We therefore determined the proinsulin to insulin ratios in the fasting state and after acute stimulation of insulin secretion in 23 postmenopausal women, aged 61-62 yr (mean +/- SD, 61.7 +/- 0.5 yr). Ten women had normal glucose tolerance (NGT), and 13 had IGT. The groups were matched for insulin sensitivity and did not differ in body mass index. Proinsulin and insulin secretion were measured after arginine stimulation (5 g, i.v.) at three glucose levels (fasting, 14 mmol/L, and >25 mmol/L), and the acute insulin (AIR(arg)) and proinsulin responses (APIR(arg)) were calculated as the mean 2-5 min postload increase. At fasting glucose, levels of insulin, proinsulin, or the proinsulin/insulin ratio (13.6 +/- 5.0% vs. 11.1 +/- 2.7%; P = NS) did not differ between NGT and IGT. Although the AIR(arg) values were decreased in the IGT group at all glucose levels (P < 0.05), the absolute proinsulin levels and the APIRs(arg) were similar between IGT and NGT women. Therefore, the IGT women had higher proinsulin/insulin ratios at 14 mmol/L (10.7 +/- 4.4% vs. 6.4 +/- 1.8%; P = 0.006) and more than 25 mmol/L glucose (11.4 +/- 5.2% vs. 6.7 +/- 2.1%; P = 0.007). The IGT group had increased APIR(arg)/AIR(arg) at fasting (2.2 +/- 1.4% vs. 1.3 +/- 0.6%; P = 0.047) and more than 25 mmol/L glucose (3.5 +/- 1.6% vs. 2.3 +/- 0.7%; P = 0.037). We conclude that women with IGT exhibit increased relative proinsulin secretion, suggesting a defect in the intracellular proinsulin processing before diabetes develops.