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To evaluate if an increased proinsulin-to-insulin ratio (PI/I) in former gestational diabetes mellitus (GDM) subjects could be a marker for later impairment of glucose tolerance.This study is a prospective follow-up. In a previous follow-up study of former GDM subjects 3-4 years after an index pregnancy, an increased PI/I was found also in normoglycemic nonobese former GDM subjects compared with control subjects. A 75-g oral glucose tolerance test (OGTT) was performed 3 years after the first follow-up, i.e., 6-7 years after the index pregnancy in 97 of the former GDM subjects and in 23 control subjects. A 75-g OGTT according to the World Health Organization was performed. Glucose, insulin, proinsulin, and C-peptide were determined at 0, 30, 60, 90, 120, 150, and 180 min after the glucose intake.Since the first follow-up, an additional 3 in 97 (3.1 percent) and 15 in 97 (15.5 percent) of the former GDM subjects had NIDDM or impaired glucose tolerance (IGT), respectively. All control subjects still had a normal OGTT. The fasting PI/I at follow-ups 1 and 2 was significantly correlated in the former GDM subjects (r = 0.41, P less than 0.001) and in the control group (r = 0.46, P less than 0.05). There was no significant correlation between the PI/I in follow-up 1 and the fasting or 2-h glucose values at follow-up 2. If GDM subjects with a PI/I in the upper quartile in the first follow-up were compared with those with a lower PI/I, there were no significant differences in outcome of OGTT in the second follow-up.The hypothesis that an increased fasting PI/I is a marker for later development of NIDDM or IGT in former GDM subjects could not be supported.