Chemoradiotherapy has substantially improved life expectancy in young women with cancer, but these treatments often cause infertility. One method of preserving fertility is to cryopreserve ovarian tissue, with subsequent autotransplantation of the tissue after successful anticancer therapy. This study examined the long-term effect of heterotopic transplantation of cryopreserved ovarian tissue on the histology. Ovarian tissue from rats was cryopreserved using a slow-freezing protocol. After thawing, the tissue pieces were transplanted under the splenic capsule in 14 rats of the same inbred strain and remained there for 210 or 300 days. Sex cord stromal tumors, consisting mainly of granulosa cells, were found in all of the rats. Although the hormonal situation in rats cannot be directly compared to that in humans, the development of sex cord stromal tumors in this animal model may be worth considering when cryopreserved ovarian tissue is transplanted heterotopically in fertility-preserving programs for cancer patients.