A MAN in his late 40s had been in good general health. He had smoked 2 packs of cigarettes per day but had recently stopped. He had become an avid exerciser, running about 24 to 32 km and swimming 1.6 to 3.2 km each week. Paroxysmal atrial fibrillation developed and was initially controlled by the use of digitalis. However, during a 2-year period, the episodes of atrial fibrillation became more frequent and required cardioversion and trials of several other antidysrhythmic drugs. Atrial flutter was noted during an office visit with his cardiologist. The next day, he staggered and fell on a jogging path. Emergency services were called, but an attempt to resuscitate the patient was unsuccessful.