We report on a case of urinary bladder leiomyosarcoma in a 23-year-old woman, 22 years after therapy for bilateral retinoblastoma. The tumor presented with dysuria and macroscopic haematuria. Cystoscopy revealed a tumor localized in the trigonum covered by an ulcerated urothelium. The patient underwent a transvesical tumor resection. Eight months later, a second leiomyosarcoma developed in the vertex, at a site different from the previous one. A cystoscopic trans-urethral tumor resection was performed, followed by combined chemotherapy. One year later another recurrence occurred at the site of the primary resection. Open laparotomic resection of the involved bladder wall was performed. The patient remains both recurrence and metastases free after twenty months of follow-up. Molecular analysis of the peripheral blood showed rare germline point mutation in the intron 24 of the RB1 gene. FISH analysis of the tumor tissue revealed polyploid cells with relative loss of normal RB1 gene locus, indicating deletion and second hit loss of the second RB1 allele function. Along with the ten previously reported cases, this report suggests a non-random association between the hereditary retinoblastoma and urinary bladder leiomyosarcoma. Therapy with cyclophosphamide seems to be an important risk factor. Life-long surveillance for second malignancies, including bladder leiomyosarcoma is therefore mandatory in these patients.