Differentiating between early-onset bipolar disorder (BD) and attention-deficit/hyperactivity disorder (ADHD) can be difficult. Memory problems are commonly reported in BD, and forgetfulness is among the diagnostic criteria for ADHD. We compared children and adolescents with BD (n = 23), ADHD combined type (ADHD-C; n = 26), BD + ADHD-C (n = 15), and 68 healthy controls on memory tests (Digit span, Children’s Verbal Learning Test-II). Further analyses were performed on subgroups of BD (BD-I, BD-II/BD-NOS, with and without previous psychotic symptoms). All clinical groups demonstrated some problems with free recall, but the BD subgroup with a history of psychotic symptoms had a more pervasive problem that also included recognition and semantic clustering. The ADHD-C groups demonstrated the lowest performance on working memory. These data suggest that children and adolescents with BD and previously psychotic symptoms may have inefficient encoding of verbal material, whereas memory problems in ADHD-C appear more characterized by impaired free recall.