Previous studies have indicated that mucinous carcinomas of the ovary associated with extraovarian spread at the time of presentation or follow-up almost always have extensive infiltrative invasion within the primary tumor. We present four cases of stage I ovarian mucinous tumors that lacked extensive infiltrative invasion but were associated with an unexpectedly aggressive behavior. The patients were 18, 20, 41, and 45 years of age at presentation. All four of them presented with an abdominal mass or increased abdominal girth. The tumors were all stage Ia, 17 to 37 centimeters in maximal dimension, and typically multicystic with solid areas. The number of blocks per centimeter of tumor diameter was 0.65, 0.88, 0.92, and 1.0 in the four tumors, respectively, a degree of sampling within that recommended in previous studies. Clinical findings supported that they were primary tumors rather than metastatic from an occult primary tumor. On microscopic examination, the tumors all contained foci of intraepithelial carcinoma and foci of invasion as follows: expansile invasion only (two cases), expansile invasion and microinvasive carcinoma (one case), and microinvasive carcinoma only (one case). The expansile invasion was extensive in each of the three cases in which it was present. On follow-up, each patient experienced recurrent disease 7 months to 4.5 years after diagnosis, including hematogenous spread to lung and/or bone and liver in three patients. Three of four patients developed intraperitoneal spread. Three of four patients died of disease, and one patient is alive with persistent disease. Although ovarian mucinous tumors with only expansile invasion or only microinvasive carcinoma are usually associated with an excellent prognosis, this study indicates that these tumors can rarely behave in an aggressive fashion with hematogenous spread and a fatal outcome. Some of these tumors may have contained unsampled foci of infiltrative invasion. Although the optimum level of sampling in mucinous tumors remains to be determined, we recommend additional sampling of tumors in which the initial sections reveal intraepithelial carcinoma, microinvasive carcinoma, expansile invasion, or combinations thereof.