In contrast to the current view of kinetin (K, N(6)-furfuryladenine) as an unnatural and synthetic cytokinin, recently it has been identified in plant DNA and plant extract. Here we describe identification of K in human urine using chromatography/mass-spectrometry analysis for the first time. The amount of kinetin in urine taken from unhealthy patients lung carcinoma was established to be 0.5 ng in 20 ml and a 100-fold reduced amount in healthy subjects. Since this rare base is a potential source of structural constrains it has to be removed from DNA by enzymatic DNA-repair reactions. It seems that the presence of kinetin in human is linked to oxidative damage processes.