Serum neopterin/creatinine ratios were longitudinally measured in 86 renal transplant recipients from the day before transplantation until 4 months after transplantation, and the relationship to the clinical symptoms of cytomegalovirus (CMV) infection was studied. Infection with cytomegalovirus occurred in 23 patients, 11 cases of which were due to primary infection. Symptoms caused by CMV infection were more severe in male patients, in patients who had received prior antirejection treatment, and in patients with primary CMV infection. The measurement of serum neopterin/creatinine ratios proved to be a marker for the severity of symptoms caused by CMV infection, as peak values were significantly higher in eight patients with CMV disease than in patients with no or only mild symptoms of CMV infection (P < 0.05). Moreover, in seven out of eight cases of CMV disease, serum neopterin/creatinine ratios started to rise up to 2 weeks before CMV infection was proven by serology.