Objectives. To determine the lifetime prevalence of 18 mental disorders and to establish the pattern that those disorders have with the age of onset in five cities of Peru. Materials and methods. As part of the World Mental Health Survey, the study in Peru followed a probabilistic multistage sample of people between 18 and 65 years old in Lima, Chiclayo, Arequipa, Huancayo and Iquitos. The desktop version of the Composite International Diagnostic Interview (CIDI) was administered. Results. The lifetime prevalence of at least one mental disorder was 29% (SE 1.2), and the prevalence of at least two or three was 10.5% (SE 0.7) and 4% (SE 0.4), respectively. Anxiety disorders were more common with 14.9% (SE 0.9) prevalence, followed by mood disorders with 8.2% (SE 0.5), impulse control disorders with 8.1% (SE 0.8), and substance use disorders (5.8%; SE 0.3). The age of onset was earlier for anxiety disorders (15 years old) and for impulse control disorders (20 years old). Younger respondents were more likely to have a mental disorder. Conclusions. Almost a third of the adult population of five cities in Peru has had some psychiatric disorder at a given time in their lives, and comorbidity is common. Most disorders begin before age 30.