The presence or absence of tubal mucosal epithelial proliferation (MEP) and its degree of severity when present were investigated in 191 cases, including 49 ovarian serous tumors of low malignant potential (LMP), 33 nonserous (predominantly mucinous) LMP tumors, various other benign and invasive malignant female genital tract lesions, and 37 cases (73 fallopian tubes) of tubal ligation for sterilization. MEP including all degrees of severity was found to be an almost ubiquitous (83% of all cases) lesion of no clinical significance. On the other hand, MEP of moderate or marked degree was rarely seen (3% of cases) in normal tubes obtained for sterilization, but was identified in from 25% to > 40% of tubes accompanying a wide variety of nonneoplastic and neoplastic lesions. Thus, we recommend that only moderate to marked MEP be considered as a diagnosable lesion in the future. There was no tendency in this material for MEP to show any specific association with ovarian serous LMP tumors, as was previously reported. However, when MEP was present, it was more likely to be widespread and mitotically active when it accompanied an ovarian LMP tumor of any histologic type than when it was found with benign ovarian lesions, invasive gynecologic cancers, uterine leiomyomas, or tubal inflammatory lesions and ectopic pregnancies.