Owing to considerable physical, endocrinological and metabolic adaptations, the analysis of biochemical data in elite and top-class athletes requires caution. With the aim to identify metabolic and biochemical adaptations to particular lifestyle conditions, such as regular and strenuous physical exercise, we measured the concentration of serum albumin, creatinine, uric acid and glucose in 80 male professional cyclists, 37 male members of the Italian national cross-country ski team and 60 male healthy sedentary controls at rest. At variance with earlier investigations, endurance athletes showed significantly decreased concentrations of serum creatinine (controls: 83.1±11.0 μmol/l; skiers: 78.0±8.4 μmol/l; p<0.05; cyclists: 73.8±10.4 μmol/l; p<0.01), uric acid (controls: 362±69 μmol/l; skiers: 331±70 μmol/l; p<0.05; cyclists: 312±61 μmol/l; p<0.01) and glucose (controls: 5.35±0.54 mmol/l; skiers: 4.94±0.41 mmol/l; p<0.01; cyclists: 4.94±0.42 mmol/l; p<0.01). The concentration of serum albumin was also decreased in athletes, but the difference did not reach statistical significance (controls: 4.76±0.26 g/l; skiers: 4.71±0.22 g/l; p=0.384; cyclists: 4.68±0.22 g/l; p=0.393). Results of the present investigation demonstrate that values of laboratory testing lying outside conventional reference limits calculated on sedentary populations might express physiological adaptations to regular and demanding physical aerobic activity, emphasizing the need for the estimation of reliable reference limits in elite and professional athletes, to avoid equivocal interpretation of results within clinical and anti-doping contests.