Objective: To explore the associations of subtypes of adult ADHD with other psychiatric problems, stressful life events, and sex differences. Method: Odds ratios were calculated using information from 17,899 participants from a population-based survey of adult twins born in Sweden between 1959 and 1985. Results: Symptoms of attention deficit hyperactivity disorder (ADHD) were associated with an increased risk for symptoms of (odds ratio [95% confidence interval]): generalized anxiety disorder (5.6 [4.3; 6.5]), major depression (2.8 [2.4; 3.2]), bipolar disorder (8.0 [5.1; 12.6]), obsessive-compulsive disorder (3.9 [3.1; 4.9]), and alcohol dependence (2.6 [2.2; 3.1]). Symptoms of ADHD were found to be associated with an increased risk for stressful life events (1.8 [1.3; 2.4]). No significant difference in comorbidity was observed between the two sexes. Conclusion: Both women and men with ADHD are at increased risk for symptoms of other psychiatric disorders. They are also at increased risk for stressful life events. (Contains 3 tables.)