The authors studied the prevalence of mitral valve prolapse (MVP) in the group of 656 children and adolescents (329 males and 327 females), who were a representative sample (obtained with the Monte Carlo method of statistical trials) of all newborns in the city of Maribor, Republic of Slovenia, in the period of 18 years (1976-1992). The results were considered positive in children and adolescents who in addition to possible history (chest pain, palpitations, dizziness, loss of consciousness, headaches, perspiration), probable auscultatory finding (mezzosystolic click and late systolic murmur), and suspected phonocardiographic and ECG findings, also had a positive M-mode echocardiographic finding. The criteria for MVP on M-mode echocardiography were taken from the literature: descending of mitral cusp, either anterior or posterior, of at least 3 mm below the line connecting points C and D. Children and adolescents were divided into six age groups (infants, toddlers, preschool children, early school age, children in puberty, adolescents). Assuming MVP as a cause of cardiac arrhythmias, beside standard ECG we also performed holter ECG monitoring in 61 children and adolescents (29 with MVP, 32 without MVP). The results were tested with standard statistical tools (chi 2-test, Student t-test, 2 x 2 Fisher chi 2-test). MVP was found in 71 patients (10.8%, 32 males and 39 females). As regards age and sex we found lower prevalence of MVP in male children (9.7%) compared to female children (11.9%). The highest prevalence was found in early school age, more so in females (14.2 vs 13.7). The differences were not statistically significant (p > 0.05). In both sexes most frequent was endosystolic prolapse (males 59.3%, females 51.3%). Most commonly both cusps are involved in the prolapse (males 78.1%, females 66.7%). Most frequently measured descending of the cusps was 3-4.5 mm (males 56.2%, females 48.7%). Negative auscultatory finding (silent MVP) was detected in 47.8% of the patients with MVP. Most patients with diagnosed MVP had no symptoms (71.8%). The prevalence of asymptomatic MVP declines with age in both sexes. The prevalence of arrhythmias, both in standard ECG and holter ECG, is higher in patients with MVP (6.8:0%--NS and 44.6%:9.3%--p < 0.05). The influence of constitutional changes (dolichostenomelia, asthenic constitution, genua valga) on the appearance of MVP is reflected in statistically significant difference in the Rohr' index in the group of patients with MVP in relation to the healthy group (p < 0.05). The higher prevalence of headache and dizziness in the group with MVP is statistically significant (p < 0.05).