Ovarian mucinous borderline tumors are divided into two morphologic groups: endocervical-like and intestinal type, the latter comprising the majority of cases. Thirty-one endocervical-like ovarian mucinous borderline tumors (ELMBTs) were reviewed and evaluated for the presence of intraepithelial carcinoma and microinvasion. Intraepithelial carcinoma was identified in 13% and stromal microinvasion in 23% of cases. All but 1 patient were stage I. Follow-up information was available for 21 patients; all were alive with no evidence of disease at a mean follow-up interval of 5.7 years. Six of 8 patients with ELMBT containing foci of microinvasion and/or intraepithelial carcinoma and for whom follow-up was available were alive with no evidence of disease at a mean follow-up interval of 6.6 years. These results indicate that ELMBTs, specifically those exhibiting intraepithelial carcinoma and microinvasion, are tumors associated with an excellent prognosis. The frequency of occurrence and criteria for the diagnosis of intraepithelial carcinoma and microinvasion in ELMBT are discussed.