KEYWORDS foreign body; persistent cystitis; urinary bladder stone; urinary tract infection Summary An unusual cause of a urinary tract infection (UTI) that presented in a woman who had previously undergone anti-incontinence sling surgery is presented here. A urinary bladder stone had formed on an eroded, nonabsorbable suture that had been used in the previous operation. Before identifying the underlying cause, the patient suffered from lower urinary tract symptoms and had undergone a prolonged course of antibiotic therapy for 3 years. Following removal of the suture, the UTI symptoms were rapidly resolved and only a short course of antibiotics was needed after the operation.