Neopterin and biopterin are two products of the pteridine pathway. Even though their roles and interrelationships have not been exactly clarified, neopterin is known as a biomarker of cell-mediated immunity. In this case, the highly elevated neopterin levels are parallel to the slightly elevated biopterin levels. On the other hand, the reduced form of biopterin-tetrahydrobiopterin is an essential cofactor of aromatic monoxygenases that leads to synthesis of tyrosine, tryptophan and dopamine neurotransmitters and its concentration in body fluids and tissues is maintained by the enzyme dihydropteridine reductase (DHPR). Increased numbers of activated lymphocytes can be found in peripheral blood, in the synovial fluid and synovial membranes or patients with rheumatoid arthritis (RA). Since the present study was undertaken to evaluate the role of the pteridine pathway in RA, we measured urine neopterin levels and dried blood DHPR activities in 36 patients with RA and in 20 healthy volunteers, in parallel with other clinical parameters. We found that neopterin excretion was significantly increased in RA patients compared with controls. The means were 433 +/- 216, 153 +/- 43 and 111 +/- 34 mumol/mol creatinine for patients in active stage, in remission and controls, respectively. Our results suggest that urine neopterin levels were strongly dependent on the stage and activity of RA. Either as an effect of the disease itself or of drug administration, slightly reduced DHPR activities were detected (3.484 +/- 0.304 for control, 2.974 +/- 0.255 in active stage RA, and 3.048 +/- 0.302 red cytochrome C/min/5 mm disc in remission).