We studied Error-Related Negativity (ERN) and Error Positivity (Pe) during a discrimination task in 319 unmedicated children divided into subtypes of ADHD (Not-ADHD/ Inattentive/ Combined), Learning Disorder (Not-LD/Reading/Math/Reading+Math), and Oppositional Defiant Disorder. Response-locked ERPs contained a frontocentral ERN and posterior Pe. Error-related Negativity and Positivity exhibited larger amplitude and later latency than corresponding waves for correct responses matched on reaction time. ADHD did not affect performance on the task. The ADHD/Combined sample exceeded controls in ERN amplitude, perhaps reflecting patients' adaptive monitoring efforts. Compared with controls, subjects with Reading Disorder and Reading +Math Disorder performed worse on the task and had marginally more negative Correct-Related Negativities. In contrast, Pe/Pc was smaller in children with Reading+Math Disorder than among subjects with Reading Disorder and Not-LD participants; this nonspecific finding is not attributable to error processing. The results reflect anomalies in error processing in these disorders but further research is needed to address inconsistencies in the literature.