An investigation was undertaken to determine if the incidence of third- and fourth-degree perineal tears has changed and, if so, what the predisposing factors might be. Data were analyzed for a 50-year period (1935-1985). An increased frequency of such tears was found after 1965, when mediolateral episiotomies were replaced almost entirely by midline ones. A constant combined rate of 17% for third- and fourth-degree tears was found for the last decade. When compared with a similar group without such tears, women with extensive tears were more likely to be nulliparous and teenage and to require epidural anesthesia, oxytocin and/or forceps application. When the incidence of tears was compared with that at a nearby community hospital, it was found to be higher at our university medical center. Excluding physician inexperience, the reason was the greater frequency of teenage pregnancies and more common use of epidural anesthesia and oxytocin at our hospital. Rectovaginal fistulae and anal incontinence requiring repair occurred in less than 1% of the total cases. Since midline episiotomies are now being performed often, third- and fourth-degree perineal tears will continue to be common and will depend on the patient population, physician experience and intrapartum hospital policies.