Severe congenital insulin resistance in the syndrome of leprechaunism is caused by mutations in the insulin receptor gene. We report a patient with leprechaunism who was homozygous for a mutation resulting in the absence of cell surface insulin receptors. To determine whether the receptor for Insulin‐like growth factor‐I (IGF‐I) is involved in the phenotype of leprechaunism, we studied the effect of insulin and of IGF‐I on cells from this patient.The patient had a homozygous C → T substitution at base pair 8212 in exon 12 of the insulin receptor gene, creating a premature stop codon. This nonsense mutation is in the extracellular portion of the receptor and truncates the insulin receptor proximal to its transmembrane anchor, resulting in the absence of cell surface insulin receptors. This finding indicates that complete absence of the insulin receptor is compatible with life. Secondly, DNA synthesis was studied in skin derived fibroblasts in response to increasing concentrations of either insulin or Insulin‐like growth factor ‐I (IGF‐I), and was assessed by3H‐thymidine incorporation. In this patient’s cells, both of these hormones increased3H‐thymidine incorporation, and the effect was blocked by α‐IR3, a monoclonal antibody that blocks activation of the IGF‐I receptor.These findings confirmed the absence of the insulin receptor and indicated that insulin acts here through activation of the IGF‐I receptor. These data support the contention that the phenotypic and metabolic abnormalities of leprechaunism result from the combination of lack of insulin receptor action and over‐activation by