Spinal muscular atrophy (SMA) is among the commonest degenerative disorders of the nervous system in childhood. This is an inherited autosomal recessive disease which results in premature death of anterior horn cells of the spinal cord and is manifested by progressive weakness and atrophy of skeletal muscles. Few studies have looked at the frequency of the disease in a defined population. We identified all patients diagnosed with SMA in Iceland during a 15-year period. The diagnosis is based on typical symptoms and supported by results of electromyography/nerve conduction studies and muscle biopsy. The average annual incidence was 13.7 per 100,000 live births for all types of SMA, which is similar to that reported in other population- based studies.