Objective: There are few prospective comprehensive controlled studies of the neuropsychiatric outcome in people who attempt suicide with carbon monoxide (CO). The present study aimed to evaluate this. Methods: A consecutive series of 41 adults (81% men) with CO exposure presenting over 3 years, and 20 matched controls, were evaluated with instruments to assess orientation, attention, concentration, speed of information processing, verbal memory, premorbid intellect, executive function (working memory, verbal fluency and set-switching), mood disorder, psychotic disorder, alcohol dependence and obsessive–compulsive disorder, levels of depression, hopelessness, suicide intent, anxiety, neurobehavioural function and social and interpersonal functioning. At 2 months follow-up, the neuropsychological battery was extended to include further tests of executive function (including problem-solving) and memory (including visual memory), Results: At initial assessment, control subjects showed similar levels of cognitive impairment as CO-exposed subjects (except in the case of four CO subjects with very severe impairment), but were more depressed. At 2-months follow-up, the trends were generally towards improvement in all subjects, with no between-group differences. Conclusions: Indirect effects on cognitive state may be at least as great as direct CO neurotoxicity in suicide attempters. The study did not support CO exposure exacerbating mood disorder in this sample.