To elucidate the incidence and natural history of mitral valve prolapse (MVP) during childhood, we investigated a total of 4,238 children (aged from 1 day to 15 years) classified by age into 4 groups: Group 1:1 to 28-day-old full-term normal newborns (n = 108), Group 2: 6 to 18-month-old infants (n = 391), Group 3: 6 to 7-year-old children (n = 2,801), and Group 4: 12 to 15-year-old children (n = 938). The incidence of MVP was determined by videorecorded two-dimensional echocardiography in a double-blind method twice-over. There were 109 cases diagnosed as having MVP. The incidence rates of MVP were as follows: Group 1: 0%, Group 2: 0.25%, Group 3: 2.1% and Group 4: 5.1%. Arrhythmias were detected in 49% (27/55) by Holter ECG, and by exercise stress test in 4.7% (2/43). Eighty-three (77%) of 108 cases in Groups 3 and 4, excluding the 1 case in Group 2, showed no symptoms. Ventricular premature contraction (VPC) was the most common arrhythmia, and was benign in all cases. A mid-systolic click (MSC), late systolic murmur (LSM), MSC + LSM, and a pansystolic murmur were detected in 23.1%, 3.7%, 4.6% and 5.6%, respectively. Symptoms caused by MVP increased and appeared more apparently with age. Further prospective long-term follow-up studies to adulthood are necessary.