Hard knots on the buttocks may be caused by connective tissue nevi, or by diseases such as dermatofibroma or rumbling dermatofibrosarcoma. 1. Connective tissue nevus belongs to skin misshapen tumor lesion, related to autosomal dominant inheritance. The skin lesions are firm papules or plaques of yellow, brownish-yellow, pale-white or skin color, which can occur alone or be accompanied by other diseases. The incidence site is mainly on the trunk, and the limbs can also be involved, with atypical self-conscious symptoms. 2. Dermatofibroma belongs to benign intradermal nodules, most of which are closely related to microtrauma and mosquito bites. It is usually a single lesion with slow growth, and can be manifested as hard nodules with brown or pink color. It is mainly concentrated in the limbs, followed by the trunk and face. 3. Bulging cutaneous fibrosarcoma is a nodular tumor occurring in the skin, often infiltrating into the subcutaneous fat tissue, and may infiltrate and grow. It can occur in any part of the body, and is more common in trunk and limbs. Early clinical manifestations include patchy thickening of the skin, which may be purplish red, reddish brown or normal skin color. Surgical excision is currently the mainstay of treatment.