Trauma or Contusion: This is often the most common cause of soft tissue swelling in the thigh, resulting from direct blows or injuries to the area, leading to bruising and swelling.

Cellulitis: A bacterial infection of the skin and subcutaneous tissues, which can cause swelling, redness, warmth, and pain in the affected area.
Lymphedema: Swelling caused by impaired lymph drainage, which can be primary (due to lymphatic system abnormalities) or secondary (due to surgery, infection, or other causes).
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the thigh, which can cause swelling, pain, and warmth, although it typically affects one leg.
Lipoma or Soft Tissue Tumor: Benign or malignant tumors that can grow in the soft tissues of the thigh, causing swelling.

Necrotizing Fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia), which is a medical emergency.
Compartment Syndrome: Increased pressure within a muscle compartment that can lead to muscle and nerve damage, often due to trauma, which requires urgent intervention.
Septic Arthritis or Osteomyelitis: Infections of the joint or bone, respectively, which can cause swelling, pain, and systemic symptoms, and require prompt antibiotic treatment.

Angioedema: A rare condition characterized by the rapid swelling of the skin and mucous membranes, which can be caused by allergic reactions or genetic conditions.
Sarcoidosis: A disease in which abnormal immune cells, called granulomas, form in various parts of the body, including the skin, which can cause swelling and other symptoms.
Filarial Infection: Infections caused by filarial parasites, such as those causing elephantiasis, which can lead to significant swelling and lymphedema in the affected limb.