Methylated purines and pyrimidines derived from the degradation of transfer ribonucleic acid have been shown to be excreted in abnormal amounts in the urine of patients with cancer. Recent technology developed by Gehrke and Kuo has allowed the separation and quantification of modified nucleosides in serum using reversed-phase high-performance liquid chromatography with diode-array measurement. Serum levels of ten modified nucleosides were measured in 37 normal healthy adults to establish normal values and to correlate activity with age and sex. In addition, serum levels of patients with several malignancies were measured to determine activity in these diseases. Levels of modified nucleosides in normal individuals were consistently reproducible and showed no significant variation among males versus females or with age. Patients with malignant diseases showed consistent elevations and these were highest in patients with more advanced disease. The evidence of no significant differences in the mean levels of modified nucleosides in serum with age or sex in normal adults and elevations in patients with malignancies demonstrate the potential value of modified nucleosides as cancer biomarkers.