Objective: Evaluation of hyperchromatic crowded groups in Papanicolaou (Pap) tests from women during menstruation can be a diagnostic pitfall due to similar morphological appearances with significant cervical lesions. We studied the results of p16INK4a and ProEx C on cell blocks from Pap tests during menstruation in an attempt to facilitate the differentiation. Study Design: Immunohistochemical stains for p16INK4a and ProEx C were performed on 25 cell blocks prepared from residual liquid-based cervical material with menstrual contamination. Results: Strong, diffuse, and full thickness staining pattern for p16INK4a and ProEx C was observed in cases of high-grade squamous intraepithelial lesion (HSIL) and small cell carcinoma of the cervix. The low-grade squamous intraepithelial lesion cases and cases negative for intraepithelial lesion or malignancy were negative for ProEx C, with focal staining for p16INK4a. The benign endometrial cells had either negative or focal patchy staining, which is often associated with tubal metaplasia. Conclusion: p16INK4a and ProEx C are sensitive markers for identifying significant lesions in Pap test specimens with menstrual contamination. Patchy/mosaic staining may be seen in benign endometrial tissue with tubal metaplasia, but strong, diffuse staining likely indicates HSIL or carcinoma. These findings can be helpful in interpreting hyperchromatic crowded groups in menstrual Pap specimens. Further study may be prudent, being aware of the small study group.