OBJECTIVE To study the behavior of external urethral sphincter in chronic prostatitis (CP) patient under natural filling. METHODS Twenty-one CP patients and 17 normal volunteers were involved in the study. Both the patients and volunteers underwent ambulatory urodynamic monitoring (AM) and conventional medium filling cystometry (CMG). Urodec 500 was used for AM and Menuet for CMG. AM findings from CP patients were compared with those from normal volunteers, and the results from AM were compared with those from CMG. RESULTS In AM, the resting and voiding external urethral sphincter (EUS) pressures and maximum urethral closure pressures (MUCP) were significantly higher in CP patients [(121.5 +/- 10.3) and (85. 6 +/- 3.5) cm water, respectively] than in normal volunteers [(77.6 +/- 11.4) and (10.3 + 1.6) cm water, respectively)]. CONCLUSIONS The behavioral changes of EUS in CP patients included spasm and instability of EUS, which were demonstrated using AM under natural filling; the findings were also in accord with the results of CMG.