Forty-eight patients of median age 57 years with neurogenic faecal incontinence and 44 normal subjects of median age 51 years underwent fine-wire anal sphincter electromyography and anal manometry. The median (interquartile range (i.q.r.)) internal anal sphincter (IAS) electromyogram frequency was 0.26 (0.21-0.32) Hz for patients with faecal incontinence and 0.44 (0.31-0.55) Hz for controls (P < 0.01). The median (i.q.r.) ambulatory resting pressure was 48 (34-68) cmH2O for patients and 86 (72-102) cmH2O for controls (P < 0.01) and median (i.q.r.) frequency of transient IAS relaxations 9 (7-12) and 4 (3-7) per h respectively (P < 0.05). Mid-anal pressure fell to a greater extent in patients with incontinence during these episodes of transient IAS relaxation. Rectal pressure during such relaxation did not exceed mid-anal canal pressure in controls; for the patient group, rectal pressure increased during relaxation and exceeded mid-anal canal pressure in 36 cases. Frequent abnormal episodes of IAS relaxation may cause occult faecal leakage in patients with neurogenic faecal incontinence.