Primary T-cell lymphoma of the central nervous system (CNS) is an extremely rare tumour. The Human T-cell lymphoma virus type 1 associated Adult T-cell lymphoma/leukaemia (ATLL) often involves the CNS during its course but disease limited to the CNS is exceptional. Using clinicopathological and molecular biological information we describe a case of primary CNS ATLL with infiltration of the brainstem associated with an atypical Herpes simplex encephalitis the distribution of which corresponded to that of the tumour. CNS involvement in ATLL is discussed.