Mutations in the FOXC2 gene that codes for a forkhead transcription factor are associated with primary lymphedema that usually develops around puberty. Associated abnormalities include distichiasis and, very frequently, superficial and deep venous insufficiency. Most mutations reported so far either truncate the protein or are missense mutations in the forkhead domain causing a loss of function. The haplo-insufficient state is associated with lymphatic hyperplasia in mice as well as in humans. We analyzed the FOXC2 gene in 288 patients with primary lymphedema and found 11 pathogenic mutations, of which 9 are novel. Of those, 5 were novel missense mutations of which 4 were located outside of the forkhead domain. To examine their pathogenic potential we performed a transactivation assay using a luciferase reporter construct driven by FOXC1 response elements. We found that the mutations outside the forkhead domain cause a gain of function as measured by luciferase activity. Patient characteristics conform to previous reports with the exception of distichiasis, which was found in only 2 patients out of 11. FOXC2 mutations causing lymphedema-distichiasis syndrome reported thus far result in haplo-insufficiency and lead to lymphatic hyperplasia. Our results suggest that gain-of-function mutations may also cause lymphedema. One would expect that in this case, lymphatic hypoplasia would be the underlying abnormality. Patients with activating mutations might present with Meige disease.