Attention-deficit/hyperactivity disorder (ADHD) is characterized by a lack of concentration and/or an altered activation level. People with ADHD are found to violate traffic regulations, to commit criminal offences and to be involved in traffic accidents more often than the statistical norm. Furthermore, they show more deviant behaviour and have an increased co-morbidity regarding substance abuse and dependence. Hence, this disorder is of some forensic importance. The purpose of this case study is to demonstrate that in some cases people with ADHD may show unusual effects after the consumption of THC. A 28-year-old male, who showed abnormal behaviour and seemed to be significantly maladjusted and inattentive while sober, appeared to be completely normal with a very high plasma level of THC. Performance tests conducted with the test batteries ART2020 and TAP provided average and partly above-average results in functions related to driving. Thus, it has to be taken into account that in persons with ADHD THC may have atypical and even performance-enhancing effects.