Neuromuscular disorders are common causes of weakness and hypotonia in the infantile period and in childhood. Accurate diagnosis of specific neuromuscular disorders depends first on identification of which aspect of the peripheral neuromuscular system is affected-the motor neuron in the spinal cord, the nerve root or peripheral nerve, the neuromuscular junction, or the muscle-and then on the determination of the etiology and specific clinical entity. Spinal muscular atrophy(SMA) is the most common autosomal-recessive genetic disorder lethal to infants. The three major childhood-onset forms of SMA are now usually called type I, type II and typeⅢ. Progression of the disease is due to loss of anterior horn cells, thought to be caused by apoptosis. Diagnosis is based on the course of the illness, as well as certain changes seen on nerve and muscle biopsy and electrodiagnostic studies. More recently, our understanding of the genetics of this disorder has provided a noninvasive approach to diagnosis. We report on a 3-year-old male patient with spinal muscular atrophy type II. He had progressive muscular weakness since 18 months of age. The upper arms were slightly, and the thighs moderately atrophic. There was muscle weakness of both the upper and lower limbs, being more proximal in distribution. Electromyogram revealed a neurogenic pattern.