Objectives. To estimate the annual prevalence of eighteen mental disorders, their sociodemographic correlates and the frequency of use of mental health services by individuals aged 18 to 65 in five cities of Peru. Materials and methods. The World Mental Health Survey in Peru used the Composite International Diagnostic Interview, which provides diagnoses according to DSM-IV and ICD-10 criteria. It was performed with a multistage probabilistic sample in Lima, Arequipa, Huancayo, Iquitos and Chiclayo between July 2004 and December 2005. Results. The prevalence of mental disorders in the last twelve months was 13.5%, the most frequent being anxiety (7.9%), mood (3.5%), impulse control (3.5%) and substance misuse (1.7%). The widowed, separated and divorced showed a greater risk of disorders in the last year than those who were married or partners living together. Only 32.8% of those who had severe mental health disorders in the last twelve months received any kind of treatment. Among those with moderate or mild disorders, 18.1% and 15.4% received treatment, respectively. Conclusions. More than 13 out of 100 Peruvians reported having a mental health disorder in the last year. The magnitude of mental health disorders and the gap in those receiving care highlights the urgent need to direct care and resources towards the detection and timely treatment of mental diseases in Peru.