We have presented the morbid anatomy of a case of Sandhoff's disease and have attempted to outline morphologic differences which distinguish this entity from other GM2 gangliosidoses. Yet, it may be well to maintain a sceptical eye. The anatomic differences among Tay-Sachs disease and its variants are more quantitative than qualitative and are overshadowed by the similarities. For these reasons, a definitive diagnosis must rest with the biochemist. Nevertheless, if many bizarre MCB's are encountered, the enzyme defect may be expected to differ from that of classical Tay-Sachs disease and, when seen in endothelial cells, they favour Sandhoff's disease. More importantly, if prominent visceral storage is found, and especially if it extends beyond the reticuloendothelial system to involve such selective sites as the kidneys and pancreas, Sandhoff's disease should immediately come to mind. For the pathologist, it is this latter point which more readily sets apart Sandhoff's disease from related GM2 gangliosidoses.