Idiopathic retroperitoneal fibrosis (IRF) is an inflammatory and fibrosing process that can be complicated by periureteral encasement, ureteral obstruction, and subsequent renal failure if left untreated. Unfortunately, treatment is often delayed due to the nonspecific nature of the presenting signs and symptoms. Clinical, radiologic, and microscopic findings in IRF, if examined independently, are all nonspecific for its diagnosis. Rendering a diagnosis of IRF by fine-needle aspiration (FNA) requires supportive clinical and radiologic data and systematic evaluation of entities in the differential diagnosis. Herein we report 2 cases of IRF diagnosed by FNA with subsequent histologic confirmation. Smears prepared from the aspirates revealed a combination of inflammatory cells and fibrous tissue. The inflammatory component was comprised of a mixture of lymphocytes, plasma cells, and rare eosinophils and mast cells. These 2 cases represent, to our knowledge, only the second report of IRF diagnosed by FNA.