This study investigated specific single nucleotide polymorphisms (SNPs) and their association with attentional deficits and hippocampal volume in survivors of medulloblastoma brain tumors. The sample with neuropsychological assessment includes eighteen medulloblastoma survivors and eighteen age-and-sex-matched healthy controls. We hypothesized that medulloblastoma survivors with a GSTP1 polymorphism will have significantly greater deficits in attention span and smaller bilateral hippocampal volumes compared to survivors without a polymorphism and healthy controls. We did not establish the specificity of hippocampal volume loss, and our sample may have more global subcortical morphological alterations. When separating groups by sex, we found large effect sizes between males with a GSTP1 polymorphism and females with a GSTP1 polymorphism across measures of attention span, working memory span and processing speed. Females with a polymorphism performed significantly worse than females without a polymorphism on full-scale intelligence quotient (IQ) and verbal IQ. Sex-specific genetic risk may explain part of the variability in long-term cognitive outcomes for medulloblastoma survivors.