Inclusion Criteria:

• Adult male or female (≥ 18 years of age).
• Patients with severe chronic venous insufficiency of the lower limbs (C4a,b,c - C5 according to the CEAP (Clinical manifestations, Etiology, Anatomic distibution, Pathophysiology) classification (2020 revision) for at least one of the legs) confirmed by venous Doppler ultrasound of the lower limbs.
• Patients with an indication for a thermal cure exclusively focused on phlebology.
• Patients available to participate in a 3-week consecutive thermal cure at the Saint-Eloy Thermal Center, followed by 6 months of clinical follow-up, in accordance with the protocol schedule.
• Patients available to attend the visits scheduled in the protocol and able to complete the data collection documents.
• For women of childbearing age: patient using effective contraception.
• Patient affiliated with a health insurance plan or beneficiary (Art. L.1121-8-1, French Public Health Code).
• Patient who has given their informed consent in writing regarding their participation in the protocol.

Exclusion Criteria:

• Patients with a contraindication to thermal cure.
• Patients who have participated in thermal cure (regardless of type) within the last 9 months.
• Patients who have had an ulcer on their lower limbs that has healed within the last 3 months.
• Patients without venous trophic disorders of the lower limbs, and in particular patients with isolated class C3 chronic venous insufficiency or less in both lower limbs.
• Patients with an open ulcer on a lower limb (class C6).
• Patients for whom interventional venous treatment is due to begin within the next 6 months.
• Patients scheduled to undergo orthopedic surgery on the lower limbs within the next 6 months.
• Patients who are unable to walk.
• Patients with a known allergy to sulfur.
• Patients with any medically significant findings or significant history that may impact safety, interpretation of results, and/or subject participation in the study at 6 months, in the investigator's opinion.
• Patients simultaneously participating in another clinical trial or who have participated in another clinical trial for which the exclusion period has not ended.
• Vulnerable persons (Art. L.1121-5 to 8, and L1122-1-2, French Public Health Code).