This study shows that conventional-UDS overestimates the amount of patients diagnosed with hypocontractile or acontractile bladder. Patients with reduced contractility on ambulatory-UDS have a lower chance of SNM success. Hence, ambulatory-UDS allows us to select patients with a real acontractile bladder and predict SNM failure. In patients with storage dysfunction, additional ambulatory-UDS does not seem to contribute in predicting SNM outcome.