Twenty-four patients with isolated congenital heart block were investigated by 24-hour Holter monitoring at an average age of 9.3 +/- 5.5 years. Six patients were symptomatic and 18 were asymptomatic. Eight asymptomatic patients underwent exercise stress tests and an atropine test was performed in 10 asymptomatic patients to evaluate the capacity to accelerate the heart rate. The symptomatic patients were older than the asymptomatic patients. None of the parameters which analyse ventricular rate were significantly different in the two groups of patients. Significant ventricular arrhythmias (Lown Grade 2 or over) were recorded in 1 symptomatic and 3 asymptomatic patients. The incidence of these ventricular arrhythmias increased with age and degree of bradycardia. The percentage increase in ventricular rate after atropine correlated with what was observed on effort (r = 0.95, p = 0.01) but there was no relationship between the ventricular rates during these two tests and those recorded on Holter monitoring. The results of this series of children with isolated congenital heart block show the Holter parameters cannot distinguish symptomatic from asymptomatic patients. The exercise stress and atropine tests gave very similar results but their prognostic value has not yet been established.