Cardiac troponin T (cTnT) and troponin I (cTnI) are highly sensitive and specific for detecting myocardial damage even in the presence of skeletal muscle injury. In this study, we assessed whether ultraendurance exercise induced cardiomyocyte injury using plasma cTnT and cTnI measurements, quantitative echocardiographic wall-motion analysis, and ejection fraction measurement in athletes who participated in the Hawaii Ironman Triathlon. Twenty-three athletes (11 men) who completed the triathlon (3.9 km swim, 180.2 km bike, and 42.2 km run) participated in this study. Blood samples were obtained 2 days before and immediately after the triathlon for the determination of cTnT (Enzymun, Roche Diagnostics) and cTnI (Dade Behring) concentrations. Quantitative echocardiographic wall motion analysis and ejection fraction were obtained on 12 of the 23 participants before and immediately after the race. No subject had detectable cTnT or cTnI or abnormal echo score before the race. Following the race, 2 subjects (9%) had marked increases in both cTnT (0.15 and 0.33 microg/L) and cTnI (2.09 and 4.44 microg/L). Four additional subjects (17%) had moderate increases in cTnT (0.04 to 0.05 microg/L) but no detectable cTnI. Race time correlated inversely with cTnT (r = -0.65, p <0.01). Mean change in the number of abnormal echo segments after the race was 6.5 in those with a marked increase in cTnT and cTnI, 2.3 in those with a moderate increase in cTnT, and 1.7 in those with no increase. Ejection fraction decreased by an average of 24% after the race (p <0.002). Thus, ultraendurance exercise may cause myocardial damage as indicated by biochemical cardiac-specific markers and echocardiography. The cellular nature of this damage and whether it is transient or permanent is unclear at present.