Although there are many studies available investigating internalizing and externalizing behavior in childhood and adolescent manifestations of attention-deficit/hyperactivity disorder, there is limited information about their relevance in adults featuring persistence of the disease. We examined a large sample of 910 adults affected with attention-deficit/hyperactivity disorders (AADHD) for internalizing and externalizing behavior. Regarding correlates of internalizing behavior, AADHD probands showed significantly higher scores of the anxiety- and depression-related personality traits Neuroticism and Harm Avoidance, compared with reference values. The lifetime comorbidity of depressive disorders, anxiety disorders, and anxious or fearful Cluster C personality disorders (PDs) is elevated in AADHD patients compared with general population. Regarding correlates of externalizing behavior, patients affected with AADHD show significantly lower scores of Conscientiousness and significantly higher scores of Novelty Seeking than the published German reference values. Emotional, dramatic, or erratic Cluster B PDs were most frequent in AADHD. Internalizing and externalizing behavior notably affected psychosocial status to a similar extent. The frequency of both internalizing and externalizing behavior in AADHD might reflect an underlying emotional regulation disorder.