An actinomycotic retroperitoneal infection usually occurs in the presence of an intrauterine device (IUD). It can result in pelvic inflammatory disease and diffuse retroperitoneal fibrosis. A 39-year-old patient was admitted to the emergency unit with left flank pain. A computed tomography scan of the abdomen showed bilateral hydroureteronephrosis and a retroperitoneal malignant mass. Other tumors were excluded with a colonoscopy and an upper gastrointestinal endoscopy. Results of a fine needle aspiration biopsy showed fibrosis compatible with retroperitoneal mesenteritis. Double-J stents were placed in both ureters, and immunosuppressive therapy was started. The patient had clinical and radiologic responses to the therapy. A bilateral ureterolysis and sigmoid colon resection were done. The pathology report showed fibrosis and Actinomyces israelii infection. Parenteral and oral penicillins were administered. The probability of an Actinomyces infection in patients with retroperitoneal fibrosis should be kept in mind, especially in cases in which the patient has an intrauterine device.