The aim of the present study was to compare the values of seventeen hematologic and biochemical parameters between juvenile and adult athletes and non-athletes of both sexes. 579 athletes and 241 non-athletes participated in the study. We measured packed-cell volume, hemoglobin, red blood cell count, white blood cell count, platelet count, iron, glucose, urea, triacylglycerols, total cholesterol, high-density lipoprotein cholesterol, total/high-density lipoprotein cholesterol ratio, calcium, magnesium, creatine kinase, as well as alanine and aspartate aminotransferases. We found significant differences according to age, sex, and physical activity in the majority of the parameters. The effect size of physical activity on most parameters was small (< 0.5); however, that on packed-cell volume, glucose, urea, calcium, magnesium, and creatine kinase was moderate to high (0.5 to 0.8). It is remarkable that three of the highest effect sizes of physical activity appeared on parameters thought to be under tight homeostatic control (i. e., glucose, calcium, and magnesium). We conclude that physical training influences most of the biochemical parameters routinely measured in athletes, although, in some cases, its effect appears to be of limited biological importance. Therefore, clinical assessment on the basis of blood tests has to take into account not only the age and sex, but also the training status of individuals.