Adult granulosa cell tumor of the ovary is usually diagnosed at an early stage. However, most patients with advanced or recurrent disease will die of the disease due to limited treatment options. Data on the stromal p16 expression of ovarian adult granulosa cell tumors are limited. The aim of this study was to analyze the immunohistochemical p16 expression in the peritumoral stroma of primary and recurrent adult granulosa cell tumors and investigate whether there were significant differences in stromal p16 expression among nonpathological ovaries, benign sex cord-stromal tumors, and adult granulosa cell tumors.This study included 13 and 11 cases of primary and recurrent adult granulosa cell tumors, respectively. Non-pathological ovaries and benign sex cord-stromal tumors showed negative or weak positive expression, whereas most of the adult granulosa cell tumors showed diffuse and moderate-to-strong immunostaining.Primary adult granulosa cell tumors had significantly higher stromal p16 expression levels than nonpathological ovaries and benign sex cord-stromal tumors (p<0.001). Moreover, recurrent adult granulosa cell tumors showed significantly elevated levels of stromal p16 expression compared to primary adult granulosa cell tumors (p=0.032). In contrast, the difference in stromal p16 expression between non-pathological ovaries and benign sex cord-stromal tumors was not statistically significant (p=0.522).Our observations suggest that stromal p16 expression may be involved in the development and progression of ovarian adult granulosa cell tumors.