The CAT (Comorbilidad en Adultos con TDAH) study aimed to quantify and characterize the psychiatric comorbidity at the time of diagnosis of ADHD in adult outpatients.Cross-sectional, multicenter, observational register of adults with ADHD diagnosed for the first time.In this large sample of adult ADHD (n = 367), psychiatric comorbidities were present in 66.2% of the sample, and were more prevalent in males and in the hyperactive-impulsive and combined subtypes. The most common comorbidities were substance use disorders (39.2%), anxiety disorders (23%), and mood disorders (18.1%). In all, 88.8% patients were prescribed pharmacological treatment for ADHD (in 93.4% of cases, modified release methylphenidate capsules 50:50).A high proportion of psychiatric comorbidity was observed when adult outpatients received a first-time diagnosis of ADHD. The systematic registering of patients and comorbidities in clinical practice may help to better understand and manage the prognostic determinants in adult ADHD.