BACKGROUND AND OBJECTIVES: Studies in patients with mitral valve prolapse syndrome (MVPS) have shown the coexistence of various forms of autonomic dysfunction, and so this is an area that requires further investigation. METHODS AND MATERIALS: The study group consisted of 65 patients (36 men and 29 women), aged 16 to 43 years (mean+/-SD, 28+/-8) with symptomatic, echocardiographically proven mitral valve prolapse (MVPS) who were free of other organic heart diseases and arrhythmias. In a prospective study, heart rate variability (HRV) indexes were calculated from 24-hour Holter recordings obtained during normal daily activity and plasma norepinephine was measured, and then these data were compared among the study group according to clinic heart rate (HR). RESULTS: The study group was divided into below 60 bpm (group 1: n=13), 60 - 80 bpm (group 2: n=36) and over 80 bpm (group 3: n=16) on the basis of clinic HR. These patient groups were matched with respect to age and gender. There were significant difference in SDNNIDX, rMSSD and pNN50 between these groups (p=0.005, 0.009 and 0.002 respectively), and these HRV measures correlated inversely with clinic HR (p<0.01). As clinic HR increased, there was a tendency for plasma norepinephrine concentration to also rise (p<0.05), but there was no statistical significant difference between these groups. CONCLUSION: Our data suggests that MVPS may have subgroups of HRV, reflecting cardiac autonomic modulation, according to clinic HR.