Simmons LM 1 , Wassmer E 1 , Preece MA 1 , Chakrapani A 1 1 Dept of IMD, Birmingham Children's Hosp, Birmingham, United Kingdom Background: Late infantile GM1 Gangliosidosis presents after the 1st year with neuroregression, absence of dysmorphism, and typical MRI changes affecting the thalami and basal ganglia. We present a case where diagnosis was delayed because of MRI findings of primary leucodystrophy. Case Report: The patient presented at 14 months with delayed walking & hypotonia without dysmorphism. After 2, she lost the ability to cruise, crawl and sit. Brain MRI showed typical features of leucodystrophy without abnormalities of the thalamus, basal ganglia and distal long tracts. Investigations for MLD, peroxisomal disorders and other metabolic causes were negative. Between 2.5-4 years she had progressive intellectual decline and onset of seizures, dystonia and spasticity. Further MRI scans at 4 and 5.5 years showed progressive loss of white matter with cerebral atrophy; the basal ganglia and thalami remained relatively preserved. At 8 years, she was in a near-vegetative state with spasticity, dystonia and complex epilepsy with mild dysmorphism; spinal Xray showed anterior beaking of vertebrae. Persistent oligosacchariduria and low leucocyte betagalactosidase led to confirmation of diagnosis by demonstration of 2 mutations, c75+2_3insT/c.601 C>T in the GLB1 gene. Conclusion: GM1 Gangliosidosis should be considered in the differential diagnosis of primary leucodystrophy presenting in late infancy P-378