In 30 years of follow-up of 5,209 participants in the Framingham Study, 193 men and 183 women developed atrial fibrillation (AF). Among this group, "lone" AF occurred in 32 men and 11 women free of coronary heart disease, congestive heart failure, rheumatic heart disease, and hypertensive cardiovascular disease. To determine the characteristics and prognosis of lone AF, each case was matched to controls in the remaining Framingham sample. Comparisons indicated that levels of several risk factors associated with coronary heart disease were similar between the two groups. Atrial fibrillation cases, however, had significantly higher rates of preexisting nonspecific T- or ST-wave abnormalities and intraventricular block as determined by electrocardiograms. Follow-up for new cardiovascular events indicated similar rates of coronary heart disease and congestive heart failure, but the rate of strokes was significantly greater in the lone AF group. Findings suggest that subjects with lone AF, despite similar cardiovascular risk profiles to normal controls, have a distinct preponderance of preexisting electrocardiographic abnormalities. Furthermore, contrary to general belief, lone AF is not a benign condition; it has a serious prognosis, indicating a greater need for detection and treatment.