Clues to the electrocardiographic diagnosis of subtle Wolff-Parkinson-White syndrome in children. The electrocardiographic diagnosis of Wolff-Parkinson-White syndrome (WPW) may be missed because delta waves can be subtle in children, so we examined 66 electrocardiograms from patients with proven WPW, 24 from those with questionable WPW ("subtle WPW"), and 369 consecutive electrocardiograms from control patients to identify additional clues that WPW might be present. Three features were notable in WPW: no Q wave in left chest leads (88%), PR interval less than 100 milliseconds (80%), and left axis deviation (33%). In subtle WPW these findings were similar: 79%, 67%, and 46%, respectively. By comparison, 5% of control subjects had no Q wave, 16% had a PR interval of less than 100 milliseconds, and 4% had left axis deviation (all p less than 0.001). The coexistence of two of these features was common (74%) in WPW and subtle WPW (63%) but rare (2%) in control subjects (p less than 0.001). A PR interval of less than 100 milliseconds was less specific before 1 year of age, but 89% of patients with WPW had a QRS duration of greater than 80 milliseconds versus 2% of control subjects (p less than 0.001). Obvious WPW disappeared later in 11 patients; however, left axis deviation or lack of a Q wave persisted in eight (p less than 0.01). We conclude that the diagnosis of WPW in children, even when subtle, is suggested by the presence of these four changes. Preexcitation may persist in some patients in whom overt delta waves are no longer present. 