Ovarian serous carcinoma (OSC) is the most common ovarian epithelial malignancy. Recently, a dualistic pathway of ovarian serous carcinogenesis has been proposed based on morphologic observations and molecular genetic analysis. In this scheme, low-grade OSC arises in a stepwise fashion from a benign serous cystadenoma through a usual serous borderline tumor through a micropapillary variant of serous borderline tumor. In contrast, the more common high-grade OSC arises de novo from the ovarian surface epithelium or the epithelium of cortical inclusion cysts with an as yet unrecognized precursor lesion. Although the division of OSC into low- and high-grade variants is gaining greater acceptance, and although there is accumulating molecular genetic evidence for this, there is little published information regarding a comparison of protein expression between these two types of OSC. In this study, we have investigated the immunohistochemical expression of a wide range of proteins in cases of low-grade (n = 22) and high-grade (n = 47) OSC. Antibodies used were p53, MIB1, BCL2, WT1, HER-2/neu, C-KIT, osteopontin, and survivin. For all antibodies, except MIB1, cases were scored as 0 (negative or occasional positive cells), 1+ (<10% cells positive), 2+ (10%-25% cells positive), 3+ (26%-50% cells positive), 4+ (51%-75% cells positive) or 5+ (>75% cells positive). For MIB1, the percentage of positive nuclei was calculated. There was a statistically significant higher expression of p53, MIB1, BCL2, HER-2/neu, and C-KIT in high-grade compared with low-grade OSC (P < 0.05). Thirty of 47 (64%) cases of high-grade OSC exhibited 5+ staining with p53 compared with 4 of 22 (18%) low-grade neoplasms. Twelve of 47 (26%) high-grade OSCs exhibited 5+ staining with BCL2 compared with 1 of 22 (5%) low-grade OSCs. The mean MIB1 proliferative index in high-grade OSCs was 55.4% compared with 23.0% in low-grade OSCs. Virtually all cases of both low-grade and high-grade OSCs exhibited diffuse nuclear positivity with WT1 and diffuse cytoplasmic positivity with survivin. Osteopontin expression was variable with no significant difference in expression between low-grade and high-grade OSC. Although expression of both HER-2/neu and C-KIT was significantly higher in high-grade compared with low-grade OSC, only rare cases exhibited strong positivity with these antibodies, which could be of therapeutic value in individual cases, although this would require additional molecular investigations. The significant differences in protein expression between low-grade and high-grade OSC provides further support for a different underlying pathogenesis. In particular, the differences in p53 immunoreactivity are in keeping with the observation that p53 gene mutation is more common in high-grade than low-grade OSC.