Increased neopterin concentrations and altered tryptophan degradation are observed in diseases concomitant with cellular immune activation. This may be involved in the pathogenesis of several age-related disorders such as neurodegenerative disorders, autoimmune diseases, cardiovascular system disorders and malignancies. Therefore, in the present study, the evaluation of immune system activation by determination of tryptophan degradation and serum neopterin levels was carried out in volunteers aged >= 65 and <65 years old. The kynurenine-to-tryptophan ratio was calculated to estimate indoleamine-(2,3)-dioxygenase (IDO) activity. Tryptophan levels in the elderly (53.1 +/- 1.6 mu mol/L) were lower than individuals under 65 years (61.4 +/- 2.2 mu mol/L), whereas kynurenine concentrations in geriatrics and adults were 5.0 +/- 0.2 mu mol/L and 4.3 +/- 0.2 mu mol/L, respectively (both p<0.05). The kynurenine-to-tryptophan ratio was also significantly higher in geriatrics (92.1 +/- 3.2) than adults (73.5 +/- 2.8) (p<0.05). Neopterin levels were slightly higher in geriatrics compared to adults under 65 years old (p>0.05). Effects of gender, smoking habit, pathology and drug use on measured parameters were also evaluated. In conclusion, our findings show that aging is associated with immune activation, and immune activation may be induced by the number of existing pathologies as well as the number of drugs used.