 A Phase 2 study to evaluate safety and efficacy of sildenafil taken orally to improve or resolve lymphatic malformations in children. Subjects may receive either placebo or treatment in an oral dosage with an open label extension for subjects who received placebo. The study treatment assignment will be randomized in a double blind fashion. MRI examination will evaluate change in lesion volume due to treatment. Other safety and efficacy measures will be taken through the 32-week study duration. 