Aims To estimate lifetime risk and age of onset of mental disorders in the adult general population of Belgium. Method and materials For the World Mental Health Surveys of the World Health Organization, a representative random sample of non-institutionalized inhabitants from Belgium aged 18 or older (n = 2419) were interviewed between April 2001 and June 2002. The interview took place by means of the Composite International Diagnostic Interview, version 3.0. Lifetime prevalence, projected lifetime risk, and age-of-onset of mental disorders were assessed. Results Lifetime risk for any mental disorder was 37.1%: 22.8% for mood disorders, 15.7% for anxiety disorders, and 10.8% for alcohol disorders. Median age of onset was 38 years for mood, 14 years for anxiety, and 23 years for alcohol disorders. Prevalence estimates of mood and alcohol disorders were significantly higher in the cohorts between 18 and 34 years. Conclusion This is the first study that assessed projected lifetime risk and age of onset of mental disorders in the Belgian general population. About one-third of the Belgian non-institutionalized adult population will meet the criteria for a DSM-IV mental disorder at some time during their life. Median age of onset varies from disorder to disorder and younger cohorts had higher likelihood for developing mental disorders.