Context: IGF-I deficiency may result from impairment of GH secretion or action, or from defects in IGF-I synthesis, transport, or action. Complete deficiency of the acid-labile subunit (ALS), previously described in two male patients, the only known inherited alteration in IGF-I transport, is characterized by severe circulating IGF-I and IGF binding protein (IGFBP)-3 deficiency with only mild growth retardation. Objective: Our objective was to study the characterization, at biochemical and molecular levels, of the cause for severe circulating IGF-I and IGFBP-3 deficiency in a male patient with mild growth retardation. Patients: We report an adolescent male with delayed growth and pubertal development (Tanner stage I, -2.00 SD score for height at the age of 15.3 yr), profound circulating IGF-I and IGFBP-3 deficiency, and poor response to GH treatment. Results: The index case, as well as one of his brothers, and his sister were found to be compound heterozygotes for two novel IGFALS gene mutations: C540R, a missense point mutation; and S195_197Rdup, a 9-bp duplication. The parents and youngest brother were found to be carriers for one of these two mutations. The three affected siblings had marked reduction of IGF-I and IGFBP-3 levels, undetectable serum levels of ALS, inability to form ternary complexes, and moderate insulin resistance. All of them attained a normal near-adult height (between -1.0 and -0.5 SD score), which was nonetheless lower than that of their heterozygous brother. The IGF system was only modestly affected in the heterozygous carriers. Conclusions: This study confirms the critical role of ALS in forming ternary complexes and the maintenance of normal levels of IGF-I and IGFBP-3. Insulin resistance, pubertal delay in male patients, and poor GH responsiveness seem to be frequent findings in ALS deficiency. However, haploinsufficiency of the IGFALS gene has no discernible clinical effects with only modest impact on the IGF system.