Despite the growing interest in the diagnosis of ADHD in adults, most of the knowledge in ADHD still relies on research with children and adolescents. Gender differences in adult ADHD patients were neglected for a long time and only few studies have focused this topic. The goal of this study was to investigate differences in ADHD psychopathology in male and female adults. We examined gender differences in ADHD core and associated symptoms and in personality traits in adults with ADHD. In order to discriminate between general and ADHD-specific gender differences, we compared data of adult ADHD patients with two control groups (patients with substance abuse and healthy controls). Regarding differences in ADHD core symptoms-attention problems, hyperactivity, and impulsivity-between male and female subjects, we found inconsistent results depending on the applied diagnostic instruments. Using standardized self-report, there were no gender differences regarding attention problems and hyperactivity but regarding impulsivity. Results of a semi-standardized interview (WRAADDS) according to the Utah criteria of adult ADHD showed no gender differences regarding impulsivity and hyperactivity but regarding attention problems. Moreover, differences were found between female and male healthy controls in the domains "over reactivity" and "hot temper" but not in the group of ADHD patients. Thus, it seems that gender differences in normal population were leveled by the disorder. Concerning general personality traits, some differences between male and female ADHD patients were also present in healthy controls, suggesting no ADHD-specific effect of gender. In conclusion, male and female ADHD patients seem to be more similar than different regarding ADHD-related psychopathology and general personality traits.