Recent new data in the pathogenesis of serous pelvic cancer and the introduction of serous tubal in situ carcinoma (STIC) and its precursors have raised the question that whether all primary peritoneal cancers (PPC) are in fact of tubal origin. Therefore, the present study evaluates the frequency of STIC and its precursor lesions in cases that were diagnosed as PPC using the morphologic criteria of the most recent WHO classification. The present study evaluates immunohistochemically (Ki-67 and p53 staining) the presence of STIC and its precursor lesions (p53 signature, serous tubal intraepithelial lesion [STIL]) in the completely processed Fallopian tubes of 46 consecutive PPCs. STIC was detected in 10 patients (21.7%) and p53 signature in 9 cases (19.6%). No STIL was observed. All except 1 STIC occurred at the fimbriated end of the Fallopian tube, and a bilateral involvement was detected in 2 cases. These precursor lesions were missed during the initial routine screening. Repeated staining for p53 was negative in STIC in 2 cases. STIC and p53 signature as precursor lesions of pelvic serous cancer are detected in some but not all the cases of primary serous peritoneal cancer. There might be the 2 different carcinogenetic pathways within PPC, and further studies are required to identify the source of serous cancer in cases without an STIC lesion.