Evidence links active cigarette smoking to cervical neoplasia, but much less is known about the role of passive smoking. Using a prospective cohort design, we examined personal cigarette smoking and household passive smoke exposure in relation to the risk of cervical neoplasia. Cohorts were established based on data collected on the smoking status of all household members during private censuses of Washington County, Maryland in 1963 (n = 24,792) and 1975 (n = 26,381). Using the Washington County Cancer Registry, the occurrence of cervical neoplasia in the two cohorts was ascertained from 1963-1978 and from 1975-1994. Poisson regression models were fitted to estimate the relative risk of developing cervical neoplasia associated with active and passive smoking in both cohorts. The referent category for all comparisons was never smokers not exposed to passive smoking. The adjusted relative risk and 95% confidence limits for passive smoking was 2.1 (1.3, 3.3) in the 1963 cohort and 1.4 (0.8, 2.4) in the 1975 cohort. The adjusted relative risk and 95% confidence limits for current smoking were 2.6 (1.7, 4.1) and 1.7 (1.1, 2.6) in the 1963 and 1975 cohort, respectively. The associations were in the direction of increased risk for both passive smoking and current active smoking in both the 1963 and 1975 cohorts, but were stronger in the 1963 cohort. The results of this long-term, prospective cohort study corroborate the association between active cigarette smoking and cervical neoplasia and provide evidence that passive smoking is a risk factor for cervical neoplasia.