An adult with Sandhoff disease presented with pure lower motor neuron phenotype. Twenty years later, he showed signs of upper motor neuron involvement. 25 years from the onset, his muscle weakness slightly worsened but he was fully independent in activities of daily living. GM2-gangliosidosis can manifest as a motor neuron disease with a slowly progressive course. The correct knowledge of the natural history can be really important to achieve the diagnosis, design new therapies and evaluate clinical trials.