Objectives. To assess the role of diagnostic urethrocystoscopy in the evaluation of women with idiopathic detrusor instability (DI) refractory to conventional pharmacologic management. Methods. One hundred consecutive women (mean age 62.1 ± 15.1 years) with idiopathic DI refractory to conventional pharmacologic management were prospectively enrolled. All patients underwent a meticulous evaluation, including a detailed history, urogynecologic questionnaire, micturition diary and pad test, urinalysis and culture, physical examination, and urodynamic studies. Refractory DI was defined as the lack of clinical improvement after at least 6 months of conventional drug therapy. These patients underwent additional evaluation with diagnostic urethrocystoscopy. Results. All patients had a normal urinalysis and negative cytologic findings. Diagnostic urethrocystoscopy revealed isolated bladder tuberculosis in one and transitional cell carcinoma in another. Seven other patients had bladder diverticula (only one of which was also diagnosed by sonographic examination) and 22 had mild-to-moderate bladder trabeculations. Conclusions. The absence of other alarming signs (ie, recurrent urinary tract infection, hematuria, significant residual urinary volume, positive cytologic findings, or suspicious sonographic findings) cannot confirm the lack of significant lower urinary tract abnormalities among patients with refractory DI. Diagnostic urethrocystoscopy, a simple and safe office procedure, facilitates timely diagnosis and appropriate treatment for these patients.