A group of investigators have proposed that the presence of micropapillary or cribriform patterns within ovarian serous tumors diagnosed as borderline according to World Health Organization (WHO) criteria identifies a subset of these neoplasms that are apt to be associated with invasive peritoneal implants and therefore should be designated as "micropapillary carcinoma." The authors of the current article identified 40 serous borderline tumors that showed one or both of these patterns, using the earlier investigators' published criteria for so-called micropapillary carcinoma, and compared them with 44 tumors that lacked these patterns (controls). Twenty-six patients with micropapillary tumors were aged 21 to 76 years (mean 38); 11 with cribriform tumors were aged 34 to 79 years (mean 60); and 3 with tumors having both patterns were aged 21 to 58 years (mean 38); the control patients were aged 22 to 83 years (mean 54). An advanced stage, bilaterality, and ovarian surface growth were features of the "micropapillary" tumors more often than of the control tumors. Except for a postoperative death related to sepsis, all 11 patients with Stage I tumors with either or both patterns who were followed until their death, or for at least 5 years (mean 7.9 years), survived without evidence of disease; a twelfth patient had a recent removal of recurrent pelvic tumor at 2.8 years and was alive at 3.3 years. Six of the eight patients with Stage II or III tumors with either or both patterns who were followed for at least 5 years (mean 7.5 years) survived disease-free. No deaths from tumor or progressive recurrences occurred in 27 control cases with 5-14 (mean 7.9) years of follow-up data. The two tumor-related deaths in the entire series, one from a micropapillary tumor and one from a cribriform tumor, occurred in patients who had Stage III tumors with invasive peritoneal implants. No patient with "micropapillary" tumors and noninvasive implants had progressive disease. Two women with "micropapillary" tumors and two control subjects had stable recurrent tumor or a newly developed tumor in a contralateral ovary that had been spared during the initial operation. Our findings confirm those of previous investigators that noninvasive serous tumors with a micropapillary or cribriform pattern or both patterns may be accompanied by invasive peritoneal implants more often than tumors without such patterns and that in such cases the disease is likely to be progressive and fatal. Since so-called micropapillary carcinomas lack obvious stromal invasion within the ovary, and their prognosis when they spread to the peritoneum is much closer to that associated with typical Stage II and III serous borderline tumors than to that associated with similarly staged serous carcinomas, the authors believe that this newly described category of tumors should remain as a subset within the borderline category, with a notation that their prognosis is poor if they are associated with invasive peritoneal implants.