Objective To identify the underlying cause of clinical signs in cats with non obstructive diseases of the bladder and urethra. Design Prospective case series. Sample Population 109 cats examined by the urology service of The Ohio State University's veterinary teaching hospital because of stranguria, hematuria, pollakiuria, or urination in inappropriate locations. Procedure History was obtained and a CBC, serum biochemical analyses, serologic tests for FeLV and feline immunodeficiency virus, urinalysis, bacterial culture of urine, and contrast radiography or urethrocystoscopy (females only) were performed. Results 16 cats had cystic calculi: 8 had struvite uroliths, 7 had calcium oxalate uroliths, and 1 had a urolith of unknown composition in conjunction with an anatomic defect. Anatomic defects, including diverticulae, urethral strictures, and a malpositioned urethra, were identified in 12 cats. A urinary tract infection was identified in 1 cat, and neoplasia was diagnosed in 2. One of the cats with neoplasia also had a struvite urolith. The remaining 80 cats did not have an anatomic defect, urolith, or tumor. Ten of these cats also did not have radiographic or cystoscopic abnormalities and were presumed to have a behavioral disorder. The remaining 70 cats had radiographic or cystoscopic abnormalities, and idiopathic cystitis was diagnosed. In 14 of the cats with idiopathic cystitis, results of a urinalysis were normal. Cats with idiopathic cystitis were significantly more likely to eat dry food exclusively (59%) than were cats in the general population (19%). Clinical Implications Results suggest that idiopathic cystitis occurs commonly in cats with stranguria, hematuria, pollakiurfa, or inappropriate elimination and is associated with consumption of dry foods. Contrast radiography or cystoscopy is necessary for differentiating idiopathic cystitis from behavioral disorders in some cats. ( J Am Vet Med Assoc 1997;210:46–50)