Understanding local patterns of opportunistic uptake of cervical cancer screening provides important baseline information for an efficient transition to organized screening. The objectives of this study were to estimate the prevalence of cervical cytology use at different intervals and to quantify its associated factors in an urban Portuguese population under no organized cervical cancer screening. As part of the first follow-up evaluation of a cohort of adult dwellers from Porto, 1032 women were evaluated between 2005 and 2008. Lifetime use and age at first cervical cytology, frequency of testing, socioeconomic factors, clinical history, and usual source of care were assessed using a structured questionnaire. Age-adjusted and education-adjusted odds ratios (OR) and 95% confidence intervals (95% CI) for nonuse and underutilization (over 5 years interval between tests) of cervical cytology were computed by unconditional logistic regression. Life prevalence of cervical cytology testing was 91.2%. Among ever-users, 6.7% were screened at 3-year to 5-year intervals and the prevalence of underuse was 21.4%. Older women (≥70 vs. 30-39 years) were more likely to have never been screened (OR = 32.20, 95% CI: 4.17-248.68) and more prone to underuse (OR = 25.49, 95% CI: 6.89-94.30). Underuse was less likely in the more educated (9-12 vs. 0-3 years, OR = 0.21, 95% CI: 0.08-0.52) and in women receiving private health care (private doctor vs. public health care center, OR = 0.28, 95% CI: 0.12-0.65). In conclusion, under opportunistic screening, the high life prevalence of cervical cytology use hides a small proportion of women tested at the recommended intervals and inequalities in access to and frequency of screening.