Tay-sachs disease (TSD) is a lysosomal storage disorder which has autosomal recessive inheritance and low incidence. Occurs due to hexosaminidase A (HEX-A) enzyme deficiency in α subunit. The B1 variant is resulted from an altered substrate specificity of HEX-A, wherein the mutated enzyme retains the ability to degrade the artificial substrate but not the sulfated or natural substrate in vivo. The highest incidence, 3,1 per 100.000 live births, for B1 variant of GM2 gangliosidoses has been described in Portugal, which has been suggested as the point of origin of a founder mutation seen in Brazilian patients. The case report is about a three year old male child, who presented, one year earlier, regression on gait, speech and coordination. Then, he beguns with swallow disturbance and spasticity. The child was admitted to clinical investigation and diagnostic procedures. MRI and cerebral spinal fluid were normal. The inborn metabolism errors screen for HEX-A activity was normal, but when this exam was performed in another substrate (sulfatide) it was deficient, that is called HEXA MUGS activity. This variant B1 diagnose of TSD is rare variant disease and can be performed with this test. The genetic counseling was explained to parents giving the information about 25% of chance to descendent recurrence. The importance of this case is the challenge to make a differencial diagnosis of acute ataxia in childhood and pay attention to progressive symptons presence, which other pathologies must be suspected, like metabolic diseases.