The proto-oncogene c-kit encodes a tyrosine kinase receptor, c-kit (CD117), which has been implicated in the development of a number of human malignancies. While the preferential expression of this protein has been well documented in testicular seminomas, there is little data concerning its expression in dysgerminomas of the ovary. We examined the expression of c-kit in 30 cases of ovarian dysgerminomas using immunohistohemical staining with a polyclonal anti-CD117 antibody. Staining was graded in a semiquantitative manner as follows: negative (no staining), 1+(1-10% staining), 2+(10-29% staining), 3+(30-50% staining), or 4+ (>50% staining). Of the 30 cases examined, 26 (87%) demonstrated immunoreactivity for CD117. In total, 10 (33%) demonstrated 4+ staining; 9 (30%) demonstrated 3+staining; 3 (10%) demonstrated 2+staining; 4 (13%) demonstrated 1+staining; and 4 (13%) demonstrated no staining. In conclusion, CD117 immunoreactivity was detected in 87% of ovarian dysgerminomas, a finding that correlates with previously reported frequencies of CD117 expression in seminomas (78-100%). Thus, antibodies to c-kit may be a useful diagnostic marker for ovarian dysgerminoma. Although the prognosis of patients with dysgerminoma is generally good, this receptor could potentially serve as a target for site-specific immunotherapy as an alternative and/or complement to conventional treatment options.