Endometriosis mainly affects females of child-bearing age [
1]. Endometriosis in the intestine mostly involves the rectum and sigmoid colon [
2]. Although it is not a very rare disease, its diagnosis may be very difficult because its presentation may mimic that of many other diseases, such as cancer and inflammation [
2,
3]. That is, its symptoms and imaging findings are not specific and are similar to those of these diseases. In the absence of experience, the lesion may even be misdiagnosed as adenocarcinoma under the microscope, especially on fast frozen section examination. In contrast, in some rare conditions, other lesions could be mistaken as endometriosis. Weng reported an unusual case of rectum penetration by an intrauterine device that mimicked endometriosis of the rectum [
4]. Hemorrhoids are very common anorectal lesions [
5,
6]. For women, pregnancy is a leading cause of hemorrhoids [
5]. Anal masses and hematochezia are common symptoms of hemorrhoids [
5]. In the current case, a patient with a history of hemorrhoids for 3 years came to our hospital for further diagnosis and treatment. During the digital rectal examination, in addition to anorectal hemorrhoids, a mass under the mucosa of the rectum was detected and confirmed as endometriosis by pathological examination. Without careful body examination, the hemorrhoids in this case might have masked the existence of endometriosis. Here, we focus on the diagnostic pitfalls for a relatively rare disease that presented in combination with a common disease.