Background: Although substance use disorders (SUD) and attention-deficit/hyperactivity disorder (ADHD) show significant symptomatic overlap, ADHD is often overlooked in SUD patients. Objective: The aim of the present study was to characterize aspects of attention and inhibition (as assessed by a continuous performance test [CPT]) in SUD patients with and without a comorbid diagnosis of ADHD and in healthy controls, expecting the most severe deficits in patients with a combined diagnosis. Methods: The MOXO-CPT version, which incorporates visual and auditory environmental distractors, was administered to 486 adults, including healthy controls (n = 172), ADHD (n = 56), SUD (n = 150), and combined SUD and ADHD (n = 108). Results: CPT performance of healthy controls was better than that of individuals in each of the 3 clinical groups. The only exception was that the healthy control group did not differ from the ADHD group on the Timing index. The 3 clinical groups differed from each other in 2 indices: (a) patients with ADHD (with or without SUD) showed increased hyperactivity compared to patients with SUD only and (b) patients with ADHD showed more responses on correct timing as compared with the SUD groups (with or without ADHD). Conclusion: The CPT is sensitive to ADHD-related deficits, such as disinhibition, poor timing, and inattention, and is able to consistently differentiate healthy controls from patients with ADHD, SUD, or both. Our results are in line with previous research associating both ADHD and SUD with multiple disruptions across a broad set of cognitive domains such as planning, working memory, decision-making, inhibition control, and attention. The lack of consistent differences in cognitive performance between the 3 diagnostic groups might be attributed to various methodological aspects (e.g., heterogeneity in severity, type, and duration of substances use). Our results support the view that motor activity should be considered a significant marker of ADHD.