Young women with polycystic ovary syndrome (PCOS) are at increased risk of endometrial adenocarcinoma (EAC) through chronic unopposed estrogen production. We describe the first case, to our knowledge, of grade 1 endometrioid EAC arising in the context of complex atypical endometrial hyperplasia in a 26-year-old woman with thrombophilia and PCOS who wished to retain fertility potential and was treated using a levonorgestrel-releasing intrauterine system alone. At first follow-up biopsy, a single focus of complex hyperplasia without atypia was documented. All specimens sampled during subsequent follow-up demonstrated inactive endometrium with pseudodecidual changes, and no ultrasonographic or magnetic resonance (MR) images exhibiting myometrial invasion or endoabdominal spread were observed. This successful outcome suggests that insertion of a levonorgestrel-releasing intrauterine system is a treatment option in selected young women with early-stage EAC who are not candidates for systemic therapy and who wish to maintain fertility potential. Close histologic follow-up is required, and immediate surgery is mandatory if endometrial cancer persists.