Inclusion Criteria:

   1. Written informed consent(s) for study participation and (where applicable) the use of
   the participant's images are obtained according to national regulations from the
   participant's parent(s) or guardian(s) prior to performing any study procedures.

   2. The participant is 6 months to 10 years of age at inclusion.

   3. The participant weight is at least 8kg.

   4. A diagnosis of LM or mixed venous lymphatic malformation involving the skin and
   subcutaneous tissue and at least 3cm based on clinical and radiographic criteria.

   5. LMs may benefit from systemic therapy based on clinical criteria.

   6. Females must not be pregnant or breast-feeding.

   7. If participant is a child, parent/guardian must be able to follow instructions and
   must be willing and able to ensure that the subject is present for all required study
   visits.

   8. Subject has no contraindication for use of sildenafil.

   9. LMs may involve any part of the body.

10. Subject will have normal results on screening tests (eye exam, blood tests).

11. Subject has no contraindication for MRI examinations, such as metal implants, etc.

12. Subject must not be a smoker.

Exclusion Criteria:

   1. The participant has a medically unstable health status that may interfere with his/her
   ability to complete the study.

   2. The participant presents with one or more of the following medical conditions: hepatic
   impairment; severe renal impairment; lymphedema conditions such as Milroy disease,
   Meige lymphedema, Hennekam syndrome, Njolstad syndrome, Aagenaes syndrome, and Fabry
   disease; hypotension or at risk for hypotension; seizures or history of seizures; any
   significant cardiovascular risk factors and any condition which requires participants
   to use nitric oxide donors or nitrates in any form; underlying anatomic or vascular
   risk factors for developing non-arteritic anterior ischemic optic neuropathy (NAION)
   including low ocular cup to disc ratio, age over 10, diabetes, hypertension, coronary
   artery disease, hyperlipidemia, and smoking. (Participants with Down syndrome, Turner
   syndrome, and Noonan syndrome will be considered on a case-by-case basis).

   3. The participant has received at least one of the following medications contraindicated
   in association with sildenafil within 15 days of inclusion: Alprostadil, Azole
   antifungals, Clarithromycins, Conivaptan, Delavirdine, Erythromycins, Fluvoxamine,
   Grapefruit, Imatinib, Nefazodone, Nitrates/sodium thiosulfate, Non-selective and
   selective alpha blockers, Protease inhibitors, Rufinamide, Tadalafil, Telithromycin,
   Vardenafil, Yohimbe, Yohimbine, Amifostine, Lapatinib, Warfarin

   4. The participant requires concomitant use of potent cytochrome P450 3A4 inhibitors
   (such as ketoconazole, itraconazole, erythromycin, saquinavir) or concomitant use of
   ritonavir.

   5. The patient has had extensive prior surgery or sclerotherapy to treat LM such that
   scarring may interfere with the treatment effect of sildenafil.

   6. The participant has previously been administered treatment for LMs or surgical
   procedures have been performed to remove the index LMs.

   7. Participant is currently pregnant or considering becoming pregnant in the next 20
   weeks.

   8. The participant is known to have an allergy to sildenafil.

   9. Ulcerated or currently infected LMs with pain.

10. Diagnosis of the soft tissue tumor as LM is not clinically certain.

11. The participant is participating in another clinical study.

12. The participant has a history of priapism or is diagnosed with sickle cell anemia or
   any other disorder which may predispose to priapism.

13. The investigator may declare any subject ineligible for a valid medical reason.