Comparison of clinical and electrophysiologic features of preexcitation syndromes in patients presenting initially after age 50 years with those presenting at younger ages. Although patients may develop arrhythmias due to preexcitation syndromes at any time from the prenatal period to late adulthood, presentation in late adulthood is considered uncommon and has not been well studied. From June 1981 to June 1989, 73 patients were documented to have preexcitation syndromes on the basis of electrophysiologic studies. Those whose initial arrhythmias appeared at an age greater than 50 years (group 1, n = 13) were compared with the remaining 60 patients (group 2). All group 1 patients presented in the setting of acute medical or surgical diseases (n = 7), or chronic cardiac disease (n = 6) commonly associated with middle age and often with atrial arrhythmias; only 13 group 2 patients had underlying illnesses (p = 0.0001). Almost two-thirds of group 2 patients were evaluated because of narrow complex orthodromic tachycardia or palpitations and electrocardiographic evidence of preexcitation. Wide complex tachycardia was more often a reason for referral of older patients (7 of 13 vs 11 of 60, p less than 0.05), among whom atrial fibrillation/flutter also tended to be more frequent (4 of 13 vs 11 of 60, difference not significant). The PR and QRS intervals of group 1 patients were within the normal range and differed significantly from those of group 2 patients (PR, 0.15 +/- 0.04 vs 0.11 +/- 0.03 second, p less than 0.001; QRS, 0.09 +/- 0.01 vs 0.12 +/- 0.03 second, p less than 0.001), making electrocardiographic identification of preexcitation more difficult in group 1. Several factors likely contributed. 