Introduction: Koilocytosis (cavitation of the cytoplasm due to active HPV infection) can be detected in the screening process for cervical carcinoma. Objective: To report the practice of detection of koilocytosis and (pre)neoplasia in population screening and to exploit the collected data to propose an explanation for the relationship between HPV infection and nuclear precancerous changes. Study design: Centrally collected and stored (SBBW, Leiden, the Netherlands) data from all smears of six regional pathology laboratories (I 995-2002), coded according to KO PAC (the national cervical smear coding system; S 1: normal thru S9: invasive carcinoma) were accessed. Prevalences per 100,000 smears were calculated for koilocytosis and for squamous abnormalities after stratification for country of origin of screenees. The relative risk (RR) for the ethnic (age) groups was computed by dividing the prevalence of the relevant ethnic (age) group by the prevalence of all women. Results: Surinamese women featured the highest prevalence of koilocytosis and of all squamous abnormalities. Moroccan women the lowest. The RR for koilocytosis was highest at 30 years (1.84) and lowest at 60 (0.26). RR dependence on age of S5-S9 lesions was similar. Compared to nonkoilocytotic smears, koilocytosis was 104 times more frequent in the 1,500 S4 smears, 36x more frequent in the 6,700 S2-S3 smears, and 24x more frequent in the 1,740 S5-S9 smears. In all three categories this difference is statistically significant. Conclusion: High prevalences for both koilocytosis and for preneoplasia were detected in Surinamese immigrants, however, it still does not exclude HPV infection as a confounder linked to sexual lifestyle. The presence of koilocytosis in cervical smears may serve to identify patients with an increased risk for cervical cancer and perhaps warrant more intensive surveillance than what is provided through five-yearly screening.