Among 43 patients with Laron syndrome followed in our clinic, we were able to study the carbohydrate metabolism from infancy into adult age in 30 patients. During infancy, fasting blood glucose levels were in the hypoglycemic range (mean +/- SD, 3.5 +/- 1.2 mmol/L) and increased at the end of a delayed puberty to 4.6 +/- 0.6 mmol/L. Fasting plasma insulin was higher than expected for concomitant glucose levels, and several of the 20 patients who underwent an oral glucose tolerance test (OGTT) had glucose intolerance and relatively high insulin levels. In adult patients, insulinopenia developed and one 38-year-old patient developed non-insulin-dependent diabetes mellitus (NIDDM) with subsequent need for insulin therapy. Continuous insulin-like growth factor-I (IGF-I) treatment of a pubertal patient with glucose intolerance and hyperinsulinemia normalized both responses. In conclusion, long-term IGF-I deficiency leads to insulin resistance, which is reversed by exogenous IGF-I administration.