Background: This longitudinal electrophysiological study investigated the course of multiple impaired cognitive brain functions in attention-deficit/hyperactivity disorder (ADHD) from childhood to adulthood by comparing developmental trajectories of individuals with ADHD and typically developing controls. Methods: Subjects with ADHD ("N" = 11) and normal controls ("N" = 12) diagnosed in childhood [mean age ADHD/CTRL = 10.9 years ["SD" 1.72]/10.0 years ("SD" 1.03)] were followed up after 1.1 and 2.4 years, and as young adults [ADHD/CTRL: 21.9 years ("SD" 1.46)/21.1 years ("SD" 1.29)]. At all four times, event-related potential (ERP) maps were recorded during a cued continuous performance test (CPT). We focused on residual deficits as adults, and on developmental trajectories (time and time x group effects) for CPT performance and attentional (Cue P300), preparatory (CNV: contingent negative variation) and inhibitory (NoGo P300) ERP components. Results: All ERP components developed without significant time x group interactions. Only the CNV remained reduced in the ADHD group, although 8/11 individuals no longer met a full ADHD diagnosis as adults. Cue P300 and NoGo P300 group differences became nonsignificant in early adulthood. The CNV parameters correlated with reaction time (RT) and RT-"SD". Perceptual sensitivity improved and the groups' trajectories converged with development, while RT-"SD" continued to be elevated in adult ADHD subjects. Conclusions: Attentional and preparatory deficits in ADHD continue into adulthood, and the attenuated CNV appears to reflect a particularly stable ADHD marker. Although some deficit reductions may have gone undetected due to small sample size, the findings challenge those developmental lag models postulating that most ADHD-related deficits become negligible with brain maturation. (Contains 3 figures and 1 table.)