Background. Previous epidemiological studies indicate large cross-national differences in prevalence of depression.

Methods. At 15 centres in 14 countries, 25916 primary care patients were screened for common mental disorders. A stratified random sample of 5447 primary care patients completed a baseline diagnostic assessment and 3197 completed a 12-month follow-up assessment. Psychiatric symptoms and diagnoses were assessed using the Composite International Diagnostic Interview (CIDI). Interviewer-rated disability was assessed using the Social Disability Schedule (SDS).

Results. Prevalence of current major depression varied 15-fold across centres. When centres were divided into three groups according to prevalence rates, the symptom pattern or latent structure of depressive illness was generally similar at low-, medium-, and high-prevalence centres. Depression was universally associated with disability, but this association varied significantly (t = 3·51, P = 0·0005) across centres. At higher-prevalence centres, depression was associated with lower levels of impairment. At 1 year follow-up, higher prevalence centres had both significantly higher rates of depression onset (t = 3·11, P = 0·002) and higher rates of persistence among those depressed at baseline (t = 2·49, P = 0·013).

Conclusions. Large cross-national variations in depression prevalence cannot be attributed to ‘category fallacy’ (cross-national differences in the nature or validity of depressive disorder). Use of identical measures and diagnostic criteria may actually identify different levels of depression severity in different countries or cultures. Cross-national differences in the onset and outcome of depression may reflect either true prevalence differences or differences in diagnostic threshold.