Abstract： A 64-year-old female presented with a right lumbar mass for over 20 years,gradually increasing in size in the last 5 years.Dermatological examination showed a solitary reddish-brown proliferative plaque on the right waist,approximately 3.5 cm ×4.0 cm in size,with irregular edges and a rough surface,covered by reddish-brown crusts.Pathological examination showed hyperkeratosis,irregular hyperplasia and hypertrophy of the stratum spinosum,and tumor cells were rectangular or oval,with pale eosinophilic cytoplasm,vacuolated nuclei,small nucleoli,and atypical mitosis.Most tumor cells were located in the epidermis and no tumor infiltration was seen.This case was diagnosed as malignant hidroacanthoma simplex.Treatment:local extended excision.No local recurrence or peripheral lymph node metastasis was seen in the 15-month postoperative follow-up.