Background Intrathecal administration of opioids may cause lower urinary tract dysfunction. In this study, the authors compared the effects of morphine and sufentanil administered intrathecally in a randomized double-blind fashion (two doses each) on lower urinary tract function in healthy male volunteers. Methods Urodynamic evaluation was performed before and every hour after drug administration up to complete recovery of lower urinary tract function using pressure and flow measurements recorded from catheters in the bladder and rectum. Sense of urge and urinary flow rates were assessed every hour by filling the bladder with its cystometric capacity and asking the patient to void. Full recovery was defined as a residual volume of less than 10% of bladder capacity and a maximum flow rate within 10% of the initial value. Results Intrathecal administration of both opioids caused dose-dependent suppression of detrusor contractility and decreased sensation of urge. Mean times to recovery of normal lower urinary tract function were 5 and 8 h after 10 or 30 microg sufentanil and 14 and 20 h after 0.1 or 0.3 mg morphine, respectively. This recovery profile can be explained by the spinal pharmacokinetics of both opioids. Conclusions Intrathecal opioids decrease bladder function by causing dose-dependent suppression of detrusor contractility and decreased sensation of urge. Recovery of normal lower urinary tract function is significantly faster after intrathecal sufentanil than after morphine, and the recovery time is clearly dose dependent.