PAX 8 is a transcription factor involved in the process of organogenesis of the thyroid gland, kidney, and Müllerian system. IHC stain of PAX8 has been widely used as marker for renal and Müllerian origins. Here, we present our experience of PAX8 IHC stain in pelvic and peritoneal lesion. The cases of pelvic and peritoneal lesion with PAX8 stain from last 3 years have been retrieved from SoftPath. The clinical-radiological information was obtained. The H&E and IHC stains (PAX8, calretinin et al) were reviewed. For PAX8 stain, only nuclear staining was considered positive. Stains were interpreted as negative (50%); weak or strong intensity. Several groups of benign and malignant lesions with positive PAX8 have been identified: glandular inclusions, endometriosis, renal cysts, mesothelial lesions, and carcinoma of Müllerian origin. Interestingly, some mesothelial lesions are both positive for PAX8 and mesothelioma marker (calretinin). In conjunction with clinic-morphological evaluation, PAX8 IHC stain is useful in the diagnosis of pelvic and peritoneal lesions: glandular inclusions; endometriosis; renal cysts, carcinoma of Müllerian origin. Interestingly, some mesothelioma are both positive for PAX8 and mesothelioma marker (calretinin). Thus PAX8 is not reliable in discriminating ovarian carcinomas from peritoneal mesothelioma.