We have recently reported a patient with a homozygous partial deletion of the insulin-like growth factor-I (IGF-I) gene, resulting in IGF-I deficiency, insulin resistance, and short stature. Recombinant human IGF-I (rhIGF-I) therapy has been shown to improve insulin sensitivity (Si) and growth in other causes of IGF-I deficiency. We now report results of 1 yr of rhIGF-I therapy on body composition, bone mineral density (BMD), insulin sensitivity, and linear growth in this patient. rhIGF-I therapy was initiated at age 16.07 yr (bone age, 14.2 yr), at a starting dose of 40 microg/kg daily, increasing after 3 months to 80 microg/kg daily. Body composition, BMD, markers of bone mineralization, and auxological parameters (height, weight) were measured at 0, 6, and 12 months after start of therapy. Si, acute insulin response to glucose, and glucose effectiveness were determined at baseline, 3 months, and 12 months into therapy. On IGF-I therapy, body mass index increased from 17 kg/m2 to 18.6 kg/m2. Body composition studies (dual-energy x-ray absorbtiometry) revealed an initial decrease in total body fat, from 19.9% at baseline to 15.1% at 6 months; but by 12 months of therapy, this had reversed, with an increase to 21.8%. Si, calculated using Bergman's minimal model, was substantially reduced at baseline at 1.45 x 10-4 min-1 (microU/mL) [normal value, 5.1 x 10-4 min 1 (lean adult male)]. rhIGF-I therapy resulted in a dose-related improvement of Si into the normal range (NR) (rhIGF-I dose: 40 microg/kg x day, Si = 2.06 x 10-4 min-l; rhIGF-I dose: 80 microg/kg x day, Si = 4.39 x 10-4 min-1). Baseline reduction in Si was accompanied by elevated acute insulin response to glucose, which also fell in a dose-dependent manner. Baseline BMD was severely reduced when compared with age-matched controls (-4.88 SD); however, calculation of bone mineral apparent density indicated that the true reduction in BMD was minimal. rhIGF-I therapy increased BMD by 17% and bone mineral apparent density by 7%, indicating that IGF-I has a greater effect on bone growth than bone mineralization. Bone turnover markers also increased on rhIGF-I; mean serum osteocalcin: 8.3 ng/mL pretreatment, 21.7 ng/mL after 6 months of rhIGF-I (NR for adult male, 3.4-9.1 ng/mL); mean bone specific alkaline phosphatase: 36.5 U/L pretreatment, 82.2 U/L after 6 months of therapy (NR for adult male, 15-41). Height velocity increased from 3.8 cm/yr pretreatment to 7.3 cm/yr on 80 microg/kg.day of rhIGF-I. In this patient with severe insulin resistance, therapy with rhIGF-I resulted in beneficial effects on Si, body composition, bone size, and linear growth. These results have implications for IGF-I therapy in a variety insulin resistant states.