The aim of this study was to determine the underlying pathology of women with high grade squamous intraepitheliallesion (HSIL) on cervical cytology. A total of 681 women with HSIL cytology undergoing colposcopic examination atChiang Mai University Hospital (CMUH) between January 2000 and December 2005 were evaluated for the underlyingcervical pathology. The final pathology was diagnosed from the most severe lesions obtained by punch biopsy, loopelectrosurgical procedure, cold knife conization or hysterectomy. Underlying high grade cervical lesions includingcervical intraepithelial neoplasia grade 2, 3 and adenocarcinoma in situ were noted in 502 (73.7%) women. Invasivecervical carcinoma was identified in 141 (20.7%). The remaining 38 (5.6%) had either low grade or no intraepitheliallesions. No significant difference in the prevalence of underlying high grade and invasive lesions was noted betweenwomen with cytologic diagnosis of HSIL from CMUH and other hospitals. In conclusion, northern Thai women withHSIL cytology are at significant risk of having underlying severe cervical lesions, and especially invasive carcinomawhich is detected in approximately one-fifth of the cases.