Objective: The neural network involved in inhibition of inappropriate response tendencies shares commonalities with the error-processing network, signaling failure of inhibition. Most studies on error processing in ADHD have been conducted in children using electrophysiological methods. Method: Using event-related functional magnetic resonance imaging, the authors studied 14 adults with ADHD and 12 group-matched healthy control participants while performing a modified version of a combined Eriksen Flanker-Go/NoGo-task. Results: Patients with ADHD demonstrated significantly reduced error signaling in the left inferior frontal gyrus bordering the anterior insular cortex (BA 47), computed from the contrast of unsuccessful minus successful inhibition trials. Conclusion: Hypoactivation of the left inferior frontal cortex during error signaling might represent a neurofunctional marker of a crucial prerequisite for error processing in adults with ADHD. This possibly indicates a dysfunction of the neural system that operates task-set related representations and monitoring of erroneous performances in service of ensuing posterror processing.