The present study utilized a case-control design to assess the relation between menstrual cycle factors and risk of endometriosis in women age 19-45 years. From a gynecologic specialty clinic in Western New York in 1987, we identified 104 laparoscopically confirmed cases of endometriosis. We used two control groups: (1) 100 friend controls (friends of cases), and (2) 98 medical controls (patients from the same medical practice with conditions other than endometriosis). Controls were frequency matched to cases by age. Because crude odds ratios (ORs), stratified by age (< 30 vs > or = 30 years), revealed effect modification by age, we performed all analyses by age group. We used logistic regression to calculate ORs and 95% confidence intervals (CIs), controlling for pregnancy, religion, and body mass index. Among women under age 30, using friend controls, we observed elevated risks for menstrual flow > or = 6 days per month (OR = 2.5, 95% CI = 1.1-5.9), heavy flow (OR = 2.5, 95% CI = 1.1-6.3), severe cramps (OR = 2.5, 95% CI = 1.2-6.0), increasing symptoms (OR = 6.6, 95% CI = 2.6-16.5), and tampon use > or = 14 years (OR = 3.6, 95% CI = 1.04-13.5). Risks were also elevated for women age 30 and over, but not to the same degree as among younger women.(ABSTRACT TRUNCATED AT 250 WORDS)