This paper gives a capillary electrophoretic method for the separation of 15 urinary normal and modified nucleosides from cancer patients in less than 40 min. A 500 mm x 50 microns uncoated capillary column (437.5 mm to window) was used. The effects of the voltage and the sodium dodecyl sulfate (SDS) concentration in the buffer on the separation were studied. With reproducibilities of migration times better than 1.2% (R.S.D.) and determined concentrations better than 5-25%, depending on the concentrations of nucleosides in the urine, the analytical characteristics of the method were good. Using this developed method, the concentrations of 13 normal and modified nucleosides, extracted on a phenyl boronic acid affinity chromatography column, in 25 urines from patients of 14 kinds of cancer were determined. The levels (nmol/mumol creatinine) of modified nucleosides in urines from cancer patients were increased as compared with those in normal urines.