Simple Summary: The aim of this study was to evaluate the incidence of high-grade glandular pathology in the histological cones of LEEP for CIN3 and to verify the different biological nature of the single CIN3 lesion from CIN3 coexisting with AIS. A sample of 414 women underwent LEEP for CIN3. From the histological examination of the cone, we selected 370 women with a single CIN3, 39 women with CIN3 coexisting with AIS, and 5 women with CIN3 coexisting with AC. Specific HR HPV genotypes and treatment outcome in the study groups were investigated. Women with CIN3 coexisting with AIS showed only four specific HR HPV genotypes (16, 18, 45, 33), the prevalence of genotype 18, the presence of skip lesions (9%), and occult adenocarcinoma (one case of 1A1 adenocarcinoma). Furthermore, CIN3 lesions coexisting with AIS have a higher rate of viral persistence and recurrence than single CIN3 lesions. These characteristics make CIN3 coexisting with AIS closer to the glandular pathology than to the squamous one, making hysterectomy necessary and cervical conization insufficient, which is instead considered the optimal treatment method for a single CIN3. The purpose of this study was to evaluate the incidence of AIS and AC in the histological cone of women treated for CIN3. Furthermore, through the study of the specific HR HPV genotypes, we obtained more information on the possible different nature between the single CIN3 lesion and the CIN3 coexisting with the glandular lesion. Methods. A sample of 414 women underwent LEEP for CIN3. The study sample consisted of 370 women with a CIN3 lesion alone and 44 women with a CIN3 lesion coexisting with AIS or adenocarcinoma. We studied the individual HR HPV genotypes and their frequency in the two groups under study. Furthermore, the therapeutic results and follow-ups for the population were studied on the entire study sample. Results. In patients with a single CIN3 lesion, 11 high-risk genotypes were detected; in patients with CIN3 associated with AIS or AC, only 4 different genotypes were detected (16, 18, 45, 33). Overall, the frequency of HPV 18 was significantly higher in CIN3 coexisting with AIS compared to solitary CIN3 lesions, χ2 = 27.73 (p < 0.001), while the frequency of other high-risk genotypes was significantly higher in patients with a single CIN3 than in patients with CIN3 coexisting with AIS. In our study population, mixed lesions (CIN3 coexisting with AIS), unlike their squamous counterparts (single CIN3 lesions), were characterized by skip lesions, which demonstrate more aggressive behavior and a higher rate of viral persistence and recurrence. Conclusion. A relatively high rate (10.7%) of AIS-AC was found in women treated for CIN3. Our study confirms the multifocal biological nature of the CIN3 lesion coexisting with AIS compared to the single CIN3 lesion. All this justifies the different treatments to which CIN3 lesions coexisting with AIS are addressed; in fact, the latter are treated with hysterectomy, while CIN3 is treated with conization alone. [ABSTRACT FROM AUTHOR]