Physical fitness of athletes affected by mitral valve prolapse (MVP) was examined, in order to evaluate the influence of sport activity on the natural history of the disease. Maximal workload, total workload, percentage efficiency (according to Hollmann's formula), double product of maximal cycloergometric stress test performed by 80 athletes (53 male, 27 female, mean age 23.8 yrs) with MVP were considered and compared with the same parameters obtained by 160 (120 male, 40 female) normal athletes (N) matched for age and weight. Moreover, the same ergometric data of two maximal exercise tests, performed by 30 subjects of the MVP group, a mean follow-up period of 2.5 years (range 1-6 years) were compared. No significant difference was found between MVP and N group ergometric data, except for double product, that was significantly lower in MVP group with respect to N group (P less than 0.001). Moreover, no difference was found between MVP with or without mitral regurgitation, and N. No difference was found between the first and the last ergometric test in the follow-up group. In conclusion, our results suggest that athletes with MVP have no reduction of physical fitness. Ergometric follow-up, almost in our cases, does not indicate a negative influence of physical activity on the natural history of the disease.