In order to evaluate the association between seropositivity for herpes simplex virus (type 1 and type 2) and cervical intraepithelial neoplacia (CIN), we analysed data from a population-based case-control study of CIN grade II-III which included Norwegian women aged 20 to 44 years, 94 cases and 228 controls. Our objectives were to determine if HSV-1 and/or HSV-2 seropositivity were independent risk factors for CIN, taking human papillomavirus exposure into account, and to elucidate the combined effect of HPV positivity and seropositivity for HSV In logistic regression analyses, the association between HSV-2 or HSV-1 seropositivity and CIN II-III was not explained by HPV (adjusted OR 3.0; 95%, CI 1.3-7.2 and adjusted OR 3.3; 95% CI 1.3-8.4, respectively). In analyses restricted to HPV-16 DNA-positive individuals, seropositivity for HSV-2 increased the risk of CIN (OR 11.1; 95% CI 1.2-105.7), whereas HSV-1 seropositivity was not significantly associated with CIN. In women positive for other HPV types, only HSV-1 seropositivity was associated with CIN (OR 8.5; 95% CI 1.3-55.8). In analyses of the HPV-16-seropositive individuals, neither HSV-1 nor HSV-2 seropositivity was associated with CIN. Compared to the reference group of jointly unexposed subjects, the HPV-16 DNA-positive women who were anti-HSV-2 negative had an increased risk of CIN (OR 29; 95% CI 12-67), whereas the risk in women who were both HPV-16 DNA-positive and HSV-2 was OR=247 (95% CI 31-1996). The estimate of interaction was strong, but did not reach significance, and our findings may suggest that the combined effect of the two viruses is of aetiological importance in cervical carcinogenesis. Furthermore, the results indicate that HPV DNA positivity is not sufficient to explain the sexual behaviour-associated risk of cervical neoplasia and that further studies on the role of genital HSV (type 1 as well as type 2) and other STDs are warranted.