Association of Kocher-Debre-Semelaigne syndrome-a myopathy of hypothyroidism in childhood characterized by muscular hypertrophy, with rhabdomyolysis is very rare. We present a case of Kocher-Debre-Semelaigne syndrome with rhabdomyolysis secondary to Hashimoto's thyroiditis. He had muscular symptoms simulating poly/dermatomyositis, massively elevated creatine kinase (CK) levels and high creatinine levels. All of the findings reversed on treatment of hypothyroidism. The response to the therapy strongly suggested that Kocher-Debre-Semelaigne (KDS) syndrome was the underlying etiology. Serum thyroid- stimulating hormone levels should be routinely determined in all patients with muscular symptoms and/or elevation of CK and creatinine, keeping KDS syndrome in mind.