Insulin resistance and impaired insulin secretion are thought to be the primary defects in the pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM). Disproportionately increased proinsulin relative to insulin levels are suggested to be an early indicator of a failing pancreas. We examined the relationship of fasting specific insulin, proinsulin, and 32, 33 split proinsulin concentrations, and the proinsulin: insulin ration to the risk of developing NIDDM 3.5 years later in 65-74-year-old non-diabetic Finnish subjects participating in a population-based study (n=892) on diabetes and heart disease. Altogether 69 subjects developed NIDDM over a 3.5-year follow-up (cases). The cases were compared to randomly-selected gender-matched control subjects (n=69) and control subjects matched for gender, glucose tolerance status (normal or impaired), and body mass index (n=69). There were no differences in insulin concentrations between cases and random or matched control subjects [median and interquartile range: 123 (77-154), 108 (74-143), 118 (83-145) pmol/l, p=0.271]. Random control subjects had lower proinsulin and 32, 33 split proinsulin concentrations and split proinsulin: insulin ratios compared to cases [5.7 (3.8-9.0) vs 7.3 (4.8-10.0) pmol/l, p=0.005; 7.3 (4.5-13.0 vs 10.4 (7.1-18.0) pmol/l, p=0.002; 0.073 (0.057-0.110) vs 0.097 (0.060- 0.135), p=0.003]. Matched control subjects had lower proinsulin concentrations and proinsulin: insulin ratios compared to cases [5.9 (4.0-7.7) vs 7.3 (4.8-10.0) pmol/l, p=0.019; 0.048 (0.035-0.071) vs 0.064 (0.045-0.100), p=0.008]. When cases were compared to matched control subjects a 1 SD increase in baseline proinsulin: insulin ratio was associated with a 1.37-fold risk (p=0.020) of developing diabetes. Moreover, this association was independent of fasting glucose concentration at baseline. Thus, in elderly prediabetic subjects disproportionately increased proinsulin concentration, an indicator of defective insulin secretion, is associated with conversion to diabetes over a short time period.