The current study explored the relationship between hippocampal volume and short-term/working memory (STM/WM) in children with Attention-Deficit/Hyperactivity Disorder (ADHD) and dyslexia (RD). It was hypothesized that there would be group differences in hippocampal volume and STM/WM performance, and that there would be a relationship between hippocampal volume and STM/WM performance. Results indicated that there were not significant differences in hippocampal volume across the groups. However, a trend of smaller bilateral posterior volumes was found in the RD groups compared to the non-RD groups, while a trend of smaller right anterior hippocampal volumes was found in the ADHD groups compared to the non-ADHD groups. The RD groups had significantly poorer performance on the verbal STM/WM factor than non-RD groups, while the ADHD groups were not found to differ in STM/WM performance from the non-ADHD groups. Finally, hippocampal volume was not a significant predictor of performance on the STM/WM factor scores for any of the groups; however, bivariate correlations indicated significant volume-performance relations that should be explored further.