This study determined the prevalence of atrial fibrillation at a primary health care centre in Fars province of the Islamic Republic of Iran. All ambulatory people aged > or = 50 years visiting the centre between April and October 2001 were screened for atrial fibrillation using a standard 12-lead ECG. The mean +/- SD age of participants was 64.0 +/- 8.9 years. Of 463 participants aged 50-79 years, 13 (2.8%) had atrial fibrillation (median age 74 years), significantly more women (10/230) than men (3/233). Unlike previous studies in industrialized countries, the prevalence tripled with each decade of life and increased significantly from 0.6% in the 50-59 years to 6.4% in the 70-79 years age group. With increasing longevity in some developing countries, atrial fibrillation and consequently stroke may become major health problems.