High sphincter pressures recorded in patients with fissure-in-ano have been attributed to sphincter spasm induced by wide recording assemblies. To investigate this hypothesis, anal sphincter pressure was measured using a series of perfused probes of 0.4-2 cm diameter in six men with chronic anal fissure in whom digital examination was easily tolerated. The results were compared with those from 14 normal men. The resting pressure within the anal canal exceeded the normal range in all six patients irrespective of probe size. With the smallest (0.4 cm) probe, the resting pressure was 114 +/- 17.1 cmH2O (mean +/- s.d.) in patients with fissure and 73.1 +/- 27.0 cmH2O (mean +/- s.d.) in control subjects (P less than 0.001) even 10 min after introduction of the device. The minimum residual pressure attained during inflation of a rectal balloon with 100 ml of air was higher in patients with anal fissure than controls, reaching statistical significance with the 1.0 cm probe (80.8 +/- 17.7 cmH2O versus 36.9 +/- 19.0 cmH2O, P less than 0.001). Maximum pressures recorded during a voluntary contraction of the sphincter were no higher than in control subjects. The results suggest that high resting pressures are recorded in patients with chronic anal fissures even when small probes are employed and are unlikely to be due to spasm, but probably represent a true increase in basal sphincter tone. It is proposed that elevated sphincter pressures may cause ischaemia of the anal lining and this may be responsible for the pain of anal fissures and their failure to heal.