OBJECTIVE To investigate time trends in incidence rates of first-time diagnosed attention-deficit/hyperactivity disorder (ADHD) in a nationwide sample aged 4-65 years across 16 years and identify potential contributing factors to these time trends. METHOD Incidence rates of first-time diagnosed ADHD based on ICD-10 criteria in Danish psychiatric hospitals per 100,000 person years (PY) were calculated for the total population, the 2 sexes, and 4 age groups using data from the Danish Psychiatric Central Research Registry and annual census data. Time trends and the role of contributing factors were analyzed and identified using joinpoint regression procedures by calculating annual percent changes for the time period 1995-2010. RESULTS A total of 20,281 patients were diagnosed with ADHD and incidence rates increased from 7.3 to 91.2 per 100,000 PY during the study period. Joinpoint analysis suggested that incidence rates for diagnosed ADHD rapidly increased from 1998 to 2002, peaked from 2002 to 2008, and slowed down from 2008 to 2010. Contributing factors to the observed time trends were a general increase in patients seen in psychiatry for any mental disorder and an increased awareness and recognition of ADHD in females, adolescents, and adults. CONCLUSIONS These results provide empirical data needed in the public and professional debate often based on theoretical rather than empirical arguments. Results support the notion of increasing incidence rates of diagnosed ADHD and identify that contributing factors are a general increase in the number of patients assessed in psychiatry and an increased recognition of females, adolescents, and adults with ADHD.