Objective: This study examined several questions about the diagnosis of attention-deficit/hyperactivity disorder (ADHD) in young adults using data from a childhood-diagnosed sample of 200 individuals with ADHD (age M = 20.20 years) and 121 demographically similar non-ADHD controls (total N = 321). Method: We examined the use of self- versus informant ratings of current and childhood functioning and evaluated the diagnostic utility of adult-specific items versus items from the "Diagnostic and Statistical Manual of Mental Disorders" ("DSM"). Results: Results indicated that although a majority of young adults with a childhood diagnosis of ADHD continued to experience elevated ADHD symptoms (75%) and clinically significant impairment (60%), only 9.6%-19.7% of the childhood ADHD group continued to meet "DSM-IV-TR" ("DSM", 4th ed., text rev.) criteria for ADHD in young adulthood. Parent report was more diagnostically sensitive than self-report. Young adults with ADHD tended to underreport current symptoms, while young adults without ADHD tended to overreport symptoms. There was no significant incremental benefit beyond parent report alone to combining self-report with parent report. Non-"DSM"-based, adult-specific symptoms of ADHD were significantly correlated with functional impairment and endorsed at slightly higher rates than the "DSM-IV-TR" symptoms. However, "DSM-IV-TR" items tended to be more predictive of diagnostic group membership than the non-"DSM" adult-specific items due to elevated control group item endorsement. Conclusions: Implications for the assessment and treatment of ADHD in young adults are discussed (i.e., collecting informant reports, lowering the diagnostic threshold, emphasizing impairment, and cautiously interpreting retrospective reports). (Contains 3 tables and 2 footnotes.)