Two female patients with clinical features resembling spinal muscular atrophy are introduced. Patient 1 presented with hypotonia and proximal weakness of extremities at the age of 4 months. The electromyography revealed motor neuronopathy suggestive of spinal muscular atrophy. Patient 2 presented with severe hypotonia, motor weakness, and joint contractures since birth. The muscle biopsy finding was consistent with spinal muscular atrophy. However, deletions in the survival motor neuron genes and the neuronal apoptosis inhibitor protein genes were not found in both the patients. They finally showed the clinical features against spinal muscular atrophy; epileptic seizures, cardiomyopathy, and spasticity. We measured the mitochondrial respiratory chain complex enzyme activities in cultured skin fibroblasts, whose results were suggestive of isolated complex I deficiency in both the patients. In conclusion, for the patients who have clinical features resembling SMA without any deletions in the SMA genes it should be considered a possibility of the mitochondrial respiratory chain complex I deficiency.