Prophylaxis against meningosis leukemia (ML) via intrathecal radioactive colloid gold injection was carried out for a period of three years (January 1974 - September 1976) in 21 children with acute lymphoblast leukemia (ALL). The analysis of those patients after one relatively long period of observation (28 months after the last case) reveals that ML developed as a primary isolated manifestation of a recurrence in three children, i.e. in 14.2 per cent versus 68 per cent in the control group without prophylaxis. In another child ML development is in parallel with bone marrow recurrence. The reduction of ML incidence is accompanied with a prolongation of the first remission (in 24 per cent over 2 years and 6 months) and a lengthening of survival for 50 per cent of the patients (62 per cent of the treated patients are alive two years and 4 months during the period of observation). The advantages of the prophylaxis with radioactive colloid gold is emphasized in comparison with the other prophylactic programmes.