This review addresses diagnosis of CNS involvement, incidence and treatment of CNS disease at time of diagnosis, prophylaxis and treatment of CNS relapse and risk factors for meningeal recurrence in adult acute lymphoblastic leukaemia (ALL). At the time of diagnosis meningeosis leukaemica is present in about 6% (1-10%) of the adult ALL patients with a higher incidence in ALL subgroups T-ALL (8%) and B-ALL (13 %). With the invention of early additional CNS directed therapy it no longer represents an unfavourable prognostic factor. In the absence of prophylaxis meningeal relapses occur in approximately one third of adults with ALL. A literature review including more than 4000 adult ALL patients showed for the different prophylactic treatment approaches the following CNS relapse rates: intrathecal therapy alone 13% (8-19%), intrathecal therapy and CNS irradiation 15% (6-22%), high dose chemotherapy 14% (10-16%), high dose chemotherapy and intrathecal therapy 8% (2-16%) and high dose chemotherapy, intrathecal therapy together with CNS irradiation 5% (1-12%). It became obvious that the early onset of intrathecal therapy and CNS irradiation and the continuation of intrathecal administrations throughout maintenance are essential. The most favourable results where achieved with high dose chemotherapy combined with intrathecal therapy and/or CNS irradiation. The majority of treatment regimens in adult ALL already include high dose chemotherapy in order to reduce the risk of bone marrow relapse. The outcome of patients with CNS relapse is still poor. Although a remission can be induced in the majority of patients (> 60%) it is usually followed by a bone marrow relapse and the survival is poor (< 5-10%). Bone marrow transplantation might be in adults at present the only curative approach.