Lymphedema of the lower extremities is a diagnostic challenge. Exclusion of secondary causes of limb swelling and secondary lymphedema is the initial step. Primary lymphedema is classified into idiopathic and familial (hereditary) subgroups. Hereditary lymphedema can be nonsyndromic or associated with congenital anomalies or with abnormal physical findings. A 13-year-old girl presented with unilateral lower extremity lymphedema. Her medical and family history was unremarkable. The physical examination was negative for dysmorphic features and congenital anomalies. Lymphoscintigraphy showed no evidence of lymph flow in the left lower extremity, which persisted at the delayed 2-hour image. A comprehensive clinical and family history that includes a thorough physical examination are the mainstays of the medical assessment of lymphedema in children. Isotopic lymphoscintigraphy is generally considered the gold standard for confirmation of the diagnosis. This article discusses the differential diagnosis, reviews the literature, and suggests a simplified and an updated flowchart for the classification of unilateral limb lymphedema in children.