Nine prepubertal children with Laron syndrome (6 males, 3 females) aged 0.5 to 14.6 years were treated by daily subcutaneous injections of IGF-I in doses of 150-200 micrograms/kg. All patients completed at least one year of treatment; six completed two years and five three years. During the first year, a significant increase in linear growth velocity, from a mean +/- SD of 4.7 +/- 1.3 to 8.2 +/- 0.8 cm/yr (p < 0.0001), was registered. In the second year the growth velocity was lower, but still significantly higher than before treatment. Bone maturation advanced proportionally with chronological age. A reduction in subcutaneous fat tissue was observed despite the body weight increase. There was no aggravation of the characteristic hypoglycemic episodes; on the contrary, there was a better tolerance to fasting. Significant increases in serum alkaline phosphatase, phosphorus and procollagens were registered throughout the study. In conclusion, IGF-I provides an effective replacement treatment for IGF-I-deficient children, mimicking most effects ascribed to growth hormone.