Among 226,464 ambulatory subjects who underwent medical check-ups over a 15-year period, 270 were found to have Wolff-Parkinson-White syndrome (1.2 case in 1,000). The syndrome was more frequent in men (181 cases, 1.4 p. 1,000) than in women (89 cases, 0.9 p. 1,000). 222 subjects were aged from 20 to 49 years (1.4 p. 1,000) and only 48 were between 50 and 80 years of age (0.7 p. 1,000). 197 subjects were re-evaluated: 119 (60.4 p. 100) complained of palpitations and 78 (39.6 p. 100) were asymptomatic. Palpitations began at all ages, even after 50 years, and usually proceeded in short attacks lasting a few seconds or minutes, with a mean recurrence rate of 5 attacks per annum (76.4 p. 100). This constant pattern sometimes was interrupted for months or years. Conversely, in a minority of cases (23.5 p. 100) an unexpected accentuation occurred which lasted for hours or days. As years went by, palpitations tented to decrease and disappear. The pre-excitation area and its degree of fusion with the normal ventricular activation had no influence on the origin and frequency of palpitations. In contrast, sustained tachycardia seemed to be more frequent in cases with lateral and posterior left pre-excitation. Among 270 subjects with pre-excitation syndrome, 7 died including 4 whose death was not due to a cardiac disease, 2 who died suddenly and 1 who succumbed to ventricular tachycardia after a road accident. None of these patients had an associated heart disease. These last 3 cases might contribute to alter the usually favourable prognosis of Wolff-Parkinson-White syndrome.