The present study was conducted to identify endometrial and myometrial lesions coexisting with adenomyosis, and to evaluate the clinicopathological characteristics of endometrial adenocarcinomas associated with adenomyosis. A retrospective analysis of the resected uterine specimens of 319 patients with adenomyosis admitted between January 1, 2014 and August 1, 2017 was performed. The endometrial and myometrial lesions coexisting with adenomyosis were evaluated. The clinicopathological prognostic factors, including tumor grade, myometrial invasion, lymphovascular space involvement, lymph node invasion, pathological stage and recurrence, were analysed. For data analysis, the Chi-squared test was used and a P-value of <0.05 was considered to indicate statistically significant differences. The mean age of the patients was 52.1 years. A total of 32 patients had endometrial carcinoma associated with adenomyosis. In addition to endometrioid adenocarcinoma of different grades, rare clear cell carcinoma cases were also observed. Two cases of malignant mesenchymal tumors (one low-grade endometrial stromal sarcoma and one leiomyosarcoma) were also diagnosed. Therefore, patients presenting with abnormal uterine bleeding should undergo thorough evaluation for the presence of adenomyosis and/or Iciomyoma(s). Although the cases of endometrial adenocarcinoma associated with adenomyosis generally had a good prognostic outcome, there were also rare cases of patients with agressive tumor morphology. The inflammatory and tissue response arising around the foci of adenomyosis generate a preventive mechanism against the invasion of adenocarcinomas coexisting with adenomyosis. This response is likely the primary mechanism underlying the good clinical course of these tumors. Therefore, the presence of adenomyosis may be an important factor for the determination of prognosis.