Anal fissure is often treated surgically by sphincterotomy. There is growing concern over the effects of this procedure on continence. Nitric oxide donors such as glyceryl trinitrate are thought to cause a reversible 'chemical sphincterotomy', capable of healing the fissure. Twenty-one consecutive patients with chronic anal fissure (13 women, mean age 36 years) were treated for 4-6 weeks with 0.2 per cent glyceryl trinitrate ointment applied to the fissure twice daily. Maximum anal resting pressure (MARP) was measured before and after application of the ointment at the first visit. There were 16 posterior and five anterior fissures. Mean(s.d.) MARP fell from 118.7(45.0) to 70.3(34.1) cmH2O over 20 min after application of the ointment (P < 0.001). Healing was complete in 11 patients at 4 weeks and in 18 at 6 weeks. The fissure recurred in four patients after cessation of treatment; three were successfully treated by further glyceryl trinitrate. Mild headache occurred in four patients. Anal fissure can be successfully treated with 0.2 per cent glyceryl trinitrate ointment applied topically.