The relation of micturition to sitting in a warm water bath was studied in 30 healthy volunteers and 21 patients with urinary retention after hemorrhoidectomy. The rectal temperature, vesical and urethral pressures as well as the EMG activity of the external urethral sphincter were recorded before sitting in and immediately after getting out of the sitz bath at 40, 45 and 50 degrees C. In the 40 degrees C bath, 16 of the 30 normal subjects reported spontaneous micturition and 14 voluntarily induced micturition. The number of spontaneous micturitions increased with higher-temperature baths. Spontaneous micturition occurred in 19 patients with urinary retention when they sat in the sitz bath; 2 patients did not micturate and were catheterized. The urethral pressure in both the normal and retention subjects showed significant reduction which increased with higher-temperature baths. There were no significant changes in the vesical pressure or EMG activity of the external urethral sphincter with water baths. Micturition on sitting in a warm water bath seems to be initiated by reflex internal urethral sphincter relaxation. A thermo-sphincter reflex is likely to be involved.