Although the neurobiological basis of the clinical entity of attention-deficit/hyperactivity disorder (ADHD) is evident, data from studies on pathomechanism-phenotype correlations are inconsistent. There are several obvious limitations of the DSM-IV diagnostic criteria to describe an adequate phenotype of adult ADHD. A dimensional model of neurobiologically based endophenotypes is therefore more likely to be compatible with the genetic model of quantitative trait loci. The primary goal of the Wuerzburg Research Initiative on Adult Attention-Deficit/Hyperactivity Disorder (WURIN-AADHD) is to test the validity of two endophenotypes, deficit in response inhibition and impairment of working memory, using various psychometric and neurobiological strategies of investigation in adult patients with ADHD. An additional objective is the investigation of the long-term course of adult ADHD. The conclusive description of valid endophenotypes of ADHD is an ongoing process that may result in a comprehensive neurobiological model for ADHD or its symptom dimensions integrating genetic, neural, cognitive, and behavioral mechanisms. This model will eventually facilitate description of complete causal connections occurring across the lifespan from early development to adulthood and is also likely to accelerate development of more specific and efficient therapeutic strategies in adult ADHD.