Background and Objective: Microscopic evaluation of cervicovaginal smears (Pap smear) plays an essential role in detection of preneoplastic and neoplastic lesions of uterine cervix. A wide spectrum of changes is seen in Pap smears. Interpretation of intermediate changes which resulting in observation of so- called ASCUS is difficult. ASCUS is defined as a cellular change that is more marked than typical reactive change but lack features diagnostic of intraepithelial neoplasia. The aim of this study was determination of reliable cytological parameters to identifying real nature of ASCUS. Patients and Methods: In this descriptive, cross sectional study, all Pap smears with presence of ASCUS, which followed by biopsy samples in Urmia, ShahidMatahari hospital during March 1999 to December 2002 were reviewed. In this regard, 17 parameters of cytoplasm, nuclei and smear background were considered and data were analyzed by SPSS and Chi Square test. Results: Four parameters of irregular nuclear contour, increased nucleo- cytoplasmic ratio, cytoplasmic vacuolization and dirty background are highly suggestive of dysplasia. Conclusion: It is important to consider that to find out the real nature of ASCUS a group of parameters are more helpful than a single one. In addition, a single Pap smear is not enough for this purpose and further investigationis required.