We report a child with ALL who presented with acute neurological collapse a
nd coning of the cerebellar tonsils as a result of massive brain infiltrati
on by leukaemic cells. Such severe involvement of cerebral parenchyma by le
ukaemic cells is rare and cerebellar tonsillar herniation has not been repo
rted, CNS parenchymal involvement with leukaemia is demonstrated using MRI
and, for the first time, cranial US.