For monitoring neopterin levels, serial serum and urinary samples were obtained from 21 renal transplant patients. In renal transplant patients, serum neopterin levels were significantly higher than in healthy volunteers, even though in a clinically stable state urinary neopterin levels were also higher than in healthy volunteers, but statistically not significantly. In cases with rejection, both serum and urinary neopterin levels were significantly more elevated than in the stable state. In contrast, serum and urinary neopterin levels were not elevated in nephrotoxicity events. These results suggest that serum and urinary neopterin levels might be valuable indicators of acute rejection. Moreover, they could be useful for differentiating acute rejection from nephrotoxicity episode.