BACKGROUND Benign symmetric lipomatosis (BSL), also known as Madelung's disease, is a rare disease generally characterized by fat deposits painlessly and symmetrically distributed in the body. In former studies, the incidence of BSL is highest in male patients and more frequent in the Mediterranean area. Asian females seem to be rarely affected. BSL often occurs in the neck or upper-back of patients, and is related to various metabolic disorders. Unusual clinical appearances may add difficulties in diagnosis. CASE REPORT In this report, we present a case of BSL in a 33-year-old woman's perineal region, with no clear BSL risk factors (sex, medical history, sites, and comorbidities), which increased the difficulties in diagnosis. The patient's quality of life was seriously affected by the continuous growth of fat tissue. Based on MRI and B-ultrasonic examinations, she underwent excision at our outpatient facility. Combined with the patient's clinical appearance, imaging results, and pathological tests, we could finally determine the diagnosis of BSL. After 18 months of follow-up, this patient recovered well with no recurrence. CONCLUSIONS Difficulties in diagnosis can seriously affect doctors' treatment approaches. BSL rarely occurs in the lower body, and our patient showed no clear risk factors. Therefore, imaging and pathological examinations can be essential tools for dermatological and plastic surgeons to diagnose and treat rare BSL.