Syndrome of extreme insulin resistance (SEIR) is a rare spectrum disorder with a primary defect in insulin receptor signalling, noted primarily in children, and is often difficult to diagnose due to the clinical heterogeneity.SEIR was diagnosed in an adolescent girl with facial dysmorphism,exuberant scalp and body hair, severe acanthosis, lipoatrophy, dental abnormalities, and short stature (Rabson-Mendenhall phenotype). She had elevated fasting (422.95 pmol/L) and post-glucose insulin levels(>2083 pmol/L). Total body fat was decreased (11%; dual-energy X-ray absorptiometry). Basal growth hormone (GH) was increased (7.9 μg/L)with normal insuline-like growth factor 1 (37.6 nmol/L) suggestive of GH resistance. She had fatty liver and polycystic ovaries. Echocardiography revealed ostium secundum type atrial septal defect (ASD). Blood glucose normalized with pioglitazone (30 mg/day). Delayed development, severe insulin resistance, mild hyperglycemia, absence of ketosis, and remarkable response of hyperinsulinemia and hyperglycemia to pioglitazone which persisted even after 1 year of diagnosis are some of the notable features of this patient. This is perhaps the first report of occurrence of congenital heart disease (ASD) in a patient of SEIR (Rabson-Mendenhall phenotype). This report highlights the clinical features of SEIR and the role of insulin sensitizers like pioglitazone in the management of such patients.