To determine the prevalence of occult anal sphincter trauma 5 years after randomization to forceps and vacuum delivery. Anal endosonography and manometry was performed in 44 of 313 women who had originally participated in one center of the Keele University Multicenter Assisted Delivery Trial at the North Staffordshire Maternity Hospital between September 1989 and May 1990. 50% admitted to defecatory symptoms and anal sphincter defects were identified in 61%. On the basis of intention to treat, 82% of forceps (n = 17) and 48% of vacuum deliveries (n = 27) had occult sphincter defects (P = 0.03). In four women, both instruments were used. However, the preponderance of defects in the forceps group persisted even when analysis was performed according to the final mode of delivery as well as in the group where only one instrument was used (n = 40). There was a significant fall in maximum squeeze anal pressure in the forceps group compared to the vacuum group (56 vs. 36 mmHg; P = 0.0007). Although twice as many in the forceps group suffered anal incontinence (32% vs. 16%) significance was not reached. Vacuum delivery appears to be associated with less occult anal sphincter trauma than forceps delivery. A large prospective randomized study is required to address the impact of specific situations, such as failed instrumentation with use of a second instrument and rotational delivery.