Twenty male adults with ADHD, 16 dyslexic adults, 15 comorbid adults, and 16 normal controls were compared on performance and underlying brain responses, during a cued Continuous Performance Test (O-X CPT), with the aim of discovering features of information processing differentiating between the groups. The study evaluated both cue- and target-related processes by analysing performance measures (errors, reaction time, and variability of reaction time), and event-related potentials (ERPs). Cue-related ERP components included the Cue-N2, Cue-P3, contingent negative variation (CNV) consisting of the CNV1, related to cue orienting, and the CNV2, related to response preparation. For targets, a distinction was made between response-related (Go), and inhibitory (Nogo) processing. Target-related components included the Go-P3, Nogo-N2, and Nogo-P3. Performance deficits were found only for the ADHD group, who demonstrated a faster decline in response speed with time-on-task and greater overall within-subject variability. No group differences were found for cue-related ERP components. Yet, controlling for group differences in internalising problems, inhibitory control was reduced in all clinical groups compared to controls, as demonstrated by an absence of frontal amplification of P3 in the Nogo condition, relative to the Go condition. For the ADHD group, in contrast to the comorbid and the dyslexic group, this effect remained after controlling for externalising symptoms, indicating that only for the ADHD group deficiencies in inhibitory control were not explained by externalising behaviour.