We have investigated 52 ECG parameters in 1,299 athletes and 151 control subjects. Data on fitness were available in 840 athletes. They were split in three groups according to level of fitness and compared with each other and the control group with regard to the ECG findings. The most marked findings with increasing fitness were lower heart rate, increased prevalence of bradycardia, increased precordial ST segment elevation, and increased T wave amplitudes. Only slightly increased parameters of right and left ventricular hypertrophy were found. A highly significant correlation was found between fitness and heart rate, while no such correlation was found between fitness and the Sokolow index. The athletes were subdivided according to sports activity, and ECG parameters were compared in endurance athletes, ball players, strength athletes, gymnasts, and controls. Apart from a lower heart rate in endurance athletes and ball players as compared with strength athletes and gymnasts, few differences were found between the athletic groups. The heart rate is the most important parameter reflecting level of fitness and sports activity. The differences in ECG findings are relatively minor and do not distinguish type of sport activity.