Idioventricular rate, QRS width, site of block and responses to ventricular overdrive pacing were studied in 29 patients with chronic complete heart block in an attempt to distinguish patients liable to syncopal attacks. Ten patients were asymptomatic, 5 gave a history of presyncope and 14 of syncope. Although the mean idioventricular rate of the 10 patients in the narrow QRS group (40.0 per min) was significantly faster than that of the 19 patients in the wide QRS group (35.5 per min; P less than 0.02), the rate did not distinguish symptomatic patients in either the narrow or the wide QRS groups. His bundle studies of the site of block were also unhelpful. Overdrive right ventricular pacing at increasing rates was used to assess ventricular escape times which were not significantly different in symptomatic and asymptomatic wide QRS patients. There was, however, a significant difference in the product of maximum ventricular escape time X overdrive cycle length between symptomatic and asymptomatic narrow QRS patients after 30 s overdrive (3.850+/-1.670 vs 1.070+/-0.475; P less than 0.01); 60 s overdrive (5.020+/-2.170 vs 1.240+/-0.515; P less than 0.01) and 120 s overdrive (6.040+/-2.900 vs 1.460+/-0.275; P less than 0.01) which may have clinical predictive values.