The phenotypic entities of primary lymphedema vary in age of onset, site of edema, associated features, inheritance patterns, and underlying genetic cause. Determining the representative phenotype for different types of genetically determined primary lymphedema has been successfully achieved with Milroy's disease and the lymphedema-distichiasis syndrome. Here we describe and illustrate their well-delineated phenotypes. Phenotype characterization facilitates the identification of causative genes, as has been demonstrated with VEGFR3 and FOXC2, in Milroy's disease and lymphedema-distichiasis respectively. Other forms of primary lymphedema are discussed.