Subjects who have had treatment for acute lymphoblastic leukemia (ALL) or a brain tumor will be asked to take part in this research study. Siblings of some subjects will also be asked to take part, so that their results can be compared with those of children who have had cancer treatment. We hypothesize that children receiving more aggressive therapy will have lower white matter brain volumes and these volumes will be significantly lower than age-matched siblings. We also hypothesize that children who take methylphenidate will show improvements on teacher and parent report measures of attention and social skills.