In a postal inquiry a 1 in 15 sample of the female population aged 18 to 45 years within Uppsala County, Sweden, was interviewed with regard to the occurrence of secondary amenorrhea, use of oral contraceptives, and smoking habits. In addition, population register data on residence, marital status, and socioeconomic distribution were used. Ninety-three per cent of the sample answered. The one-year incidence rate of secondary amenorrhea of more than 3 months' duration was 3.3 per cent, as calculated on an estimated population at risk. The one-year period prevalence rate was 4.4 per cent and the point prevalence rate at the time of interview 1.8 per cent. Oral contraceptive agents were used by 37 per cent of the sample during the last 12 months. A marked covariation between use of oral contraceptives and smoking habits was found. A plausible causal relationship between previous use of oral contraceptive agents and the occurrence of secondary amenorrhea was found in about 16 per cent of the women with amenorrhea.