Purpose of investigation: To determine which factors may increase the risk that women diagnosed with CIN I may later develop CIN II-III. Methods: A prospective study of 174 women with a grade 1 intraepithelial lesion (CIN I) confirmed by biopsy, with a follow-up time of at least one year. The following factors were studied: age, HPV infection, HPV infection by a high-risk genotype, the HPV genotypes involved, coinfection by several HPV genotypes and duration of follow-up. These factors were correlated with later detection of CIN II-III by biopsy during follow-up. Statistical analysis was performed using SPSS. Results: CIN II-III was detected at the follow-up in 24 of 174 women included in the study (13.7%), in four cases by colposcopically directed biopsy and in 20 by LLETZ. Correlation of the factors studied with the incidence of CIN II-III in this group showed that the only statistically significant factors were overall HPV infection and HPV infection by genotypes 31 and 70 (Chi-square and Fisher’s test, p < 0.05, respectively), while the duration of follow-up came close to statistical significance (Student’s test, p = 0.052). Conclusion: HPV infection and duration of follow-up are predictive factors for the detection of CIN II-III in follow-up care for women with CIN I.