Background: Although elevated resting brain natriuretic peptide (BNP) concentrations reflect heart disease, the meaning of exercise-induced increases is poorly understood and has been examined in small groups only. Therefore, the present study aimed to examine the increase in N-terminal pro-brain natriuretic peptide (NT-proBNP) and relations to cardiac troponin I and T (cTnI, cTnT) elevations after prolonged strenuous exercise in a large cohort of athletes. Methods: We examined exercise-induced changes in NT-proBNP, cTnI, and cTnT in 105 obviously healthy endurance athletes (40 ± 8 years) before and after prolonged strenuous exercise. Blood samples were taken before, 15 minutes, and 3 hours after a marathon (n = 46), a 100-km run (n = 14), and a mountain bike marathon (n = 45). Results: Eighty-one of 105 athletes exceeded the upper reference limit of NT-proBNP (males/females 88:153 ng/L) after exercise. NT-proBNP increased in all 3 events (P < .001) with the highest increase in the 100-km runners (median increase 200 ng/L; 25th/75th percentile 115/770 ng/L), which differed from the increase in the marathon (97 ng/L; 36/254 ng/L) or the mountain bike marathon (78 ng/L; 37/196 ng/L) (P < .01). Cardiac troponin I exceeded 0.04 μg/L in 74%; cTnT exceeded 0.01 μg/L in 47% of athletes after exercise. NT-proBNP was not related to exercise-induced increases in cTnI or cTnT, but correlated with exercise time (r = 0.55, P < .001). Conclusions: Increases in NT-proBNP can be found in a major part of obviously healthy athletes after prolonged strenuous exercise. The release of BNP during and after exercise may not result from myocardial damage but may have cytoprotective and growth-regulating effects. The different nature of exercise-induced increases in BNP and cardiac troponins has to be elucidated in the future.