Background: Understanding the patterns of cervical cytology use in preventive care may provide useful information for an efficient transition from opportunistic screening to organized programmes. We aimed to identify the determinants of non-use and underuse of cervical cytology in Portuguese women. Methods: As part of the fourth National Health Survey (2005/2006), 2191 women aged between 25 and 64 years were evaluated. The previous use of cervical cytology was classified as never or ever, and, among the latter, those having performed the latest cytology testing >5 years before were considered to underuse cervical cytology. We assessed the determinants of non-use and underuse through age- and education-adjusted odds ratios (ORs) and corresponding 95% confidence intervals (95% CIs). Results: Overall, 23.5% of women had never used cervical cytology and 10.7% reported underuse. This prevalence increased with age and decreased with education and income. Compared with the national mean, the lowest risk of non-use and underuse was observed in Norte (non-use: OR = 0.31, 95% CI: 0.23–0.42; underuse: OR = 0.60, 95% CI: 0.40–0.91) and the highest in Alentejo (non-use: OR = 2.33, 95% CI: 1.78–3.06; underuse: OR = 2.37, 95% CI: 1.43–3.93). Women without a private health insurance (OR = 2.65, 95% CI: 1.29–5.47), who had no doctor appointments in the preceding 3 months (OR = 2.06, 95% CI: 1.22–3.48) and those who had never performed a mammography (OR = 17.78, 95% CI: 9.09–34.78) were more likely to have never performed a cervical cytology. Conclusion: This study shows inequalities in the use of cervical cancer screening in Portugal and provides useful information for a better allocation of resources for cancer screening.