Why anorectal pain is relieved by sitting in a warm-water bath was investigated in 18 healthy volunteers and 28 patients with painful anorectal disease (18 patients with fissures and 10 with hemorrhoids). Investigations consisted of measuring rectal and interstitial sphincter temperature, rectal and rectal neck pressures, and electromyographic activity of both the external and internal anal sphincters before and after the subjects sat in a warm-water bath at temperatures of 40, 45, and 50 degrees C for 10 min each time. Pain relief was more evident and lasted longer at higher bath temperatures. The rectal and interstitial sphincter temperatures were unchanged before and after bath in both the healthy volunteers and patients. The rectal neck pressure and internal and sphincter electromyographic activity dropped significantly in the bath, but increased gradually to pretest levels 25-70 min after exiting the bath. The higher the bath temperature, the greater the drop in rectal neck pressure and internal sphincter EMG activity, and the longer the time needed to return to pretest levels. Pain relief after Sitz bath seems to be the result of internal anal-sphincter relaxation with a resulting diminution of the rectal neck pressure. The relaxation of the internal sphincter following the warm bath postulates a relationship, but direct action was ruled out. A neural pathway through a "thermosphincteric reflex" seems most likely.