We interviewed a population sample of 2412 men aged 40-70 y in Brazil, Italy, Japan and Malaysia about medical history, lifestyle habits and sexual behavior. Men were classified as having moderate or complete erectile dysfunction (ED) if they reported to be sometimes or never able to achieve and maintain an erection satisfactory for sexual intercourse, respectively. There were 1335 men with no diagnosis of cardiovascular or prostate diseases, diabetes, ulcer or depression, nor taking hormones. The prevalence of ED was 16.1%. ED was associated with age (the risk increased 8% per y), moderate (odds ratio (OR)=2.2) or severe (OR=4.9) lower urinary tract symptoms and smoking (OR=2.3 for >30 cigarettes/day). It was inversely associated with physical activity (OR=0.5) and higher educational levels. Between the ages of 40 and 70 y, almost one in six 'healthy' men is affected by ED. Further research should look at preclinical disease stages and genetic factors.