The aim of our study is to investigate, whether the inhibition of adult neural stem cell proliferation in the hippocampus by intrathecal chemotherapy and/or cerebral radiation is responsible for treatment induced memory deficits. We will investigate patients suffering from acute lymphatic leukaemia (ALL) that receive prophylactic intrathecal chemotherapy and brain irradiation. The study represents a longitudinal investigation including a virtual "humanized" version of the morris-water-maze to test hippocampus dependent spatial memory, as well as MR-imaging for morphological (volumetry) and biochemical (spectroscopy) data.