Increasing numbers of adult attention-deficit hyperactivity disorder (ADHD) patients are reported. Co-occurring disorders are reported, and treatment with pharmacological stimulants (PS) increases. The aim of this study was to analyse the incidence of adult ADHD in Denmark between 1995 and 2012 and the diagnostic stability and the diagnoses of patients contraindicating treatment with PS. This was a nationwide register-based study of patients diagnosed with ADHD aged ≥ 18 years between 1995 and 2012. The study included 6,701 patients. Incidence increases exponentially; 40.4 % of all patients had another psychiatric diagnosis before being diagnosed with ADHD. Afterwards, 17.4 % received other diagnoses. Diagnoses contraindicating PS were found in 25.8 % of the patients with other diagnoses before (10.5 % of total) and in 40.0 % (6.9 % of total) after a diagnosis of ADHD. There is an increasing incidence and instability in the diagnosis of ADHD. Patients receive diagnoses contraindicating treatment with PS.