The correct identification of invasive implants in the peritoneum in serous borderline tumors (SBTs) of the ovary is an important determinant of diagnosis, treatment, and prognosis. Although the histologic criteria to distinguish noninvasive from invasive implants have been defined, the distinction can still be difficult. We examined the presence and distribution of mesothelial cells, stromal fibrocytes, and myofibroblasts in invasive and noninvasive peritoneal implants in 100 noninvasive, 100 invasive, and 100 metastatic nests/foci from 20 cases of SBTs with peritoneal implants, 10 serous carcinomas with peritoneal metastasis, and 10 cases of endosalpingiosis by immunostaining for calretinin, CD34, and alpha-SMA. All 100 invasive nests from seven SBTs and all 100 metastatic nests from the cases of serous carcinoma showed loss of calretinin+ mesothelial cells and stromal CD34+ fibrocytes around the nests. In contrast, 72/100 noninvasive nests displayed the presence of mesothelial cells around the nests and 68 displayed preservation of surrounding stromal fibrocytes. Alpha-smooth muscle actin positive myofibroblasts were present as a stromal response in 100/100 metastatic nests, 100/100 invasive nests and 54/100 noninvasive nests. The loss of mesothelial cells and stromal fibrocytes surrounding invasive nests together with a proliferation of myofibroblasts as demonstrated by immunostaining proved to be a sensitive and specific tool to separate invasive from noninvasive implants and represents an important adjunct to morphologic diagnosis. Combined sensitivity and specificity of the three antibodies was 100 and 81%, respectively. These methods, however, may not be helpful for small biopsies of noninvasive desmoplastic implants. The distribution of these cells provides some insights into the histogenesis of invasive and noninvasive implants in SBTs.