The incidence of isolated CNS-relapse in the SPOG ALL studies 1976-1986 was analyzed and the prophylaxis of meningosis leucaemica of the different studies was compared. In the SPOG ALL high-risk study 1979-1983, the incidence of isolated CNS-relapse was significantly higher (17/71, 24%) than in the other studies. In this period, radiotherapy was omitted and the prophylactic treatment consisted only of moderately high doses of intravenous methotrexate and intrathecal methotrexate. In other studies, it was shown that the prophylactic combination of CNS-radiotherapy and intrathecal methotrexate, or the periodic administration of combined intrathecal chemotherapy alone, during the whole therapy of 2 1/2 years, produced comparably good results. The prophylaxis with the combined intrathecal chemotherapy was less neurotoxic and allowed the use of a curative radiotherapy in case of a CNS-relapse.