Retroperitoneal fibrosis is a rare fibroinflammatory condition involving the abdominal aorta, iliac vessels, and ureters that carries an association with several other autoimmune conditions. Most cases of retroperitoneal fibrosis are thought to be idiopathic. The disorder can affect all age groups but is most common in persons between the ages of 50 and 70 years. A subset of cases is associated with an underlying immunohematologic abnormality including lymphoma. We describe in this case report a highly unusual presentation of a young woman who died with a diagnosis of "idiopathic retroperitoneal fibrosis" based on multiple biopsy procedures. Postmortem examination, however, revealed disseminated anaplastic lymphoma kinase-positive anaplastic large cell lymphoma. The clinical and histopathologic importance of this very unusual presentation of anaplastic lymphoma kinase-positive anaplastic large cell lymphoma with retroperitoneal fibrosis is discussed.