We describe a 12-year-old girl with a retroperitoneal lymphatic malformation (LM) and horseshoe kidney. The imaging characteristics of the lesions are reported. Retroperitoneal LM coexisting with horseshoe kidney is extremely rare. We hypothesized that they might share the similar etiologic factors. Imaging examinations are helpful in the definition of the 2 lesions and the relationship between them, but no characteristic findings are available to diagnose retroperitoneal LM before surgery. Surgical excision is ideal to treat LM, and the prognosis is good. Although asymptomatic horseshoe kidney need not be treated, it is important for patients to receive regular follow-up because of the propensity for various complications.