The regulation of circulating insulin-like growth factor-binding protein-2 (IGFBP-2) in humans is not well understood. In vitro and animal data have identified the role of insulin in the regulation of IGFBP-8, but such a relationship has not been established clearly in humans. In the present study, serum IGFBP-8 concentrations were assessed by Western ligand blot and immunoblot analysis in children with newly diagnosed and untreated insulin-dependent diabetes mellitus (IDDM) and maturity-onset diabetes of the young before and at various times after insulin therapy. For comparison, IGFBP-8 levels were also determined in lean and obese age-matched controls. Children with IDDM were grouped according to their serum bicarbonate levels at the time of presentation (group A, >20; group B, 13-20; group C, <13 milliequivalents/L). Densitometric analysis demonstrated that before insulin therapy, group A patients had serum IGFBP-8 levels comparable to those in lean controls, and no significant change in IGFBP-2 was observed during insulin therapy. However, group B patients had a 2-fold elevation in IGFBP-2 levels before insulin therapy compared to lean controls. In these patients, IGFBP-2 tended to decrease at 1 week, but was not significantly reduced until 1 month after the initiation of insulin therapy. Group C patients had a 2.5-fold elevation of IGFBP-2 before treatment, which normalized by 1 month after treatment. Children with maturity-onset diabetes of the young, who had insulin levels and body mass indexes greater than IDDM patients and lean controls, had significantly lower IGFBP-2 levels than both lean and obese controls. IGFBP-2 levels tended to decrease further during insulin therapy. These results indicate that long standing alterations in serum insulin concentrations beyond the physiological range have significant influence on serum IGFBP-2 levels in children and confirm earlier findings that serum IGFBP-2 levels are not acutely regulated by insulin.