A 2-year 9-month-old male was referred for gait disturbances. Main complaints were abnormal gait with frequent falls observed as soon as he began to walk unaided, at 18 months of age. The first neurologic examination revealed symmetric and proximal weakness in the lower limbs with difficulty running and walking upstairs. Deep tendon reflexes were decreased, and generalized hypotonia was observed. Three months later, at 3 years of age, he had lost independent gait, and 1 month later he could not stand unaided. DNA analysis revealed homozygous deletion in exons 7 and 8 of SMN1 gene, confirming the diagnosis of spinal muscular atrophy. According to the current classification, this patient would be classified as spinal muscular atrophy type III. The distinctive feature of this case was the short time elapsed (18 months) between onset of spinal muscular atrophy and the age at which he lost ambulation. This patient reinforces the notion that late onset of symptoms in spinal muscular atrophy and acquisition of independent gait do not exclude a rapidly progressive motor deterioration, which is important when talking with families about outcome. In those rapidly progressive cases, when promptly available, testing for SMN1 gene will prevent unnecessary, invasive, or uncomfortable procedures such as lumbar puncture, electromyography, or spinal cord magnetic resonance imaging.