Infertility is a common complication of pelvic inflammatory disease (PID) and may result in decreased parity. Low parity and possibly infertility are risk factors for ovarian cancer. We therefore examined the association between ovarian cancer and history of PID in a case-control study conducted during 1989-1992 in metropolitan Toronto and nearby areas of Southern Ontario, Canada. In total, 450 histologically verified new primary epithelial ovarian cancer cases ages 35-79 years were interviewed concerning their reproduction history. Over the same period, 564 randomly selected population controls, frequently matched to the cases according to three 15-year age groups, were interviewed similarly. Continuous unconditional logistic regression methods were used for analysis. It was found that cases were more likely than controls to report having had one or more episodes of PID; adjusted for age, parity, duration of oral contraceptive use, and other factors the odds ratio (OR) was 1.53 [95% confidence interval (CI), 1.10-2.13; P = 0.012]. Higher risk was present for women with recurrent PID (OR, 1.88; 95% CI, 1.13-3.12; P = 0.014). The elevated risk associated with PID was seen particularly among women < 60 years of age at interview (OR, 1.60; 95% CI, 1.09-2.35; P = 0.016), for women of parity 0 or 1 (OR, 2.40; 95% CI, 1.39-4.15; P = 0.0017), among women who had never ever had infertility (OR, 3.74; 95% CI, 1.28-10.9; P = 0.016), and for the small number of women who reported having PID before age 20 (OR, 3.08; 95% CI, 1.17-8.13; P = 0.023).(ABSTRACT TRUNCATED AT 250 WORDS)