PIP: The study was designed to investigate the epidemiology of secondary amenorrhea within a county of Sweden. By means of questionnaire data regarding the occurrence of secondary amenorrhea of more than 3 months duration were collected together with background variables such as age, marital status, pregnancy history, use of oral contraceptives, and smoking habits. Subjects consisted of 2000 women of a total population of 30,000 women of age group 18-45 years in Uppsala county. From a computer list of all women born on day 10 or 20 of every month a 1 in 15 sample of the female population in the age group 18 to 45 years was obtained. Eventually 93.1% answered the questionnaire. Women who reported amenorrhea of more than 3 months duration in the past year were sent a second questionnaire asking for information regarding social and medical circumstances considered to be of interest. 258 women (13.8%) of the 1862 gave a history of amenorrhea of more than 3 months duration during the past year. The amenorrhea was due to pregnancy in 183 women (9.8%), secondary to surgical treatment in 13 women (.7%). 62 women, 3.3% were considered to have secondary amenorrhea. No case of primary amenorrhea was found. The age at menarche made no significant difference. 65% of the total group had borne a child. 23% were using oral contraceptives, 1/3 of the youngest and 1/10 of the oldest. 1/2 of the women under age 35 were cigarette smokers as compared to 1/3 of those above age 35. Of the factors considered; age, marital status, residence (city or rural), smoking habits and pregnancy history, only the age factor was found to be significant (t = 2.07; p less than .05). A marked covariation between the use of oral contraceptives and smoking habits was found. A plausible causal relationship between previous use of oral contraceptives and the occurrence of secondary amenorrhea was found in about 16% of the women with amenorrhea. However, findings indicate that the true incidence of secondary amenorrhea due to previous use of oral contraceptive agents is very low, .7% in the present study.