The objective of this study was to determine the association between vaginal Chlamydia infection and cervical intraepithelial neoplasia (CIN). Data were collected in a case–control study for 60 patients with CIN in biopsy and 85 control subjects with normal colposcopy and biopsy. Serum antibodies to C. trachomatis were associated with an increased risk for CIN [odds ratio (OR) = 7.3; 95% confidence interval (CI) 1.5–35.2)]. There was also a significant association between presence of inclusion bodies for C. trachomatis and CIN (OR = 5.5; 95% CI 2.4–12.4). These results indicate a strong association between CIN and chlamydial cervicitis.