It is often difficult to discern exercise-induced changes in laboratory parameters from pathological changes in elite athletes during the regular training process. This is particularly true in soccer players who compete more often than many other athletes. Therefore, soccer players of the first two German soccer leagues (all professional athletes) are recruited for repeated blood sampling during the season (4 sampling times, one of them immediately before training commences in the summer, the other ones in October/November, February/March, and April/May). Among other standardizing requirements, it is mandatory that precisely one training session (not zero, not two, no competition) takes place the day before sampling. Several confounders for the interpretation of laboratory results are verified by means of a questionnaire. Phases of high competitive stress are compared with those of low stress. The course of the parameters as well as confidence intervals for given times during the season are of main interest.