Thirty women (mean age 49 years; range 33-74) with mild to moderate pelvic relaxation and genuine stress incontinence underwent preoperative urodynamic studies. Biopsy specimens from the levator ani muscles during vaginal hysterectomy and repair were evaluated for the number and the individual diameters of type I (slow-twitch) and type II (fast twitch) fibers. Biopsies from 11 women revealed striated muscle tissue, whereas no muscle tissue was observed in the other 19. Compared with patients without muscle tissue, subjects with positive muscle findings showed significantly higher transmission ratios (55.0 +/- 23.4 versus 32.9 +/- 22.1%; P less than .05). Mean diameter and counts of types I and II muscle fibers decreased significantly with age (P less than .05). Urethral closure pressure during stress significantly increased with mean diameter of type II muscle fibers (P less than .05). In contrast to patients with severely damaged pelvic floor muscles, the presence of slow-twitch fibers is associated with improved support of the pelvic viscera, especially under conditions leading to intra-abdominal pressure rise. The presence of fast-twitch fibers improves the periurethral continence mechanism, resulting in an increase of urethral closure pressure during stress.