The Wolff-Parkinson-White syndrome has been studied in a group of healthy aviation personnel over the past 15 years. The incidence of this electrocardiographic pattern has been determined in 22,500 healthy individuals and found to be 0.25%. The prevalence of documented tachyarrhythmias in this group of individuals was found to be only 1.8% while in a group of referred patients the prevalence was 20%. The limitations of the widely accepted classification into Type A and Type B patterns was borne out by our inability to categorize 45% of subjects with the WPW pattern. Q waves as QS or QR complexes in the inferior limb leads were found in 16.7% of subjects, but in all there was Q wave-T wave vector discordance. The limited value of stress testing in these individuals was reflected by 30% of our patients who demonstrated false positive signs of ischaemic heart disease. A discussion of the incidence, classification, differential diagnosis, mechanism of tachyarrhythmias, associated cardiovascular anomalies, and treatment follows.