Cladribine is a new purine nucleoside analogue with promising activity in low-grade lymphoproliferative disorders, childhood acute myelogenous leukaemia and multiple sclerosis. Reversed phase high performance liquid chromatography and radioimmunoassay have been used for the analysis of the plasma pharmacokinetics of cladribine. The major (inactive) metabolite in plasma, chloroadenine, can only be detected by liquid chromatography. The oral bioavailability of cladribine is 37 to 51%, and that of subcutaneous administration is 100%. The terminal half-life varies from 5.7 to 19.7 hours and the apparent volume of distribution from 54 to 357 L/m2. The concentration in the cerebrospinal fluid is 25% of that in plasma in patients without central nervous system disease; in patients with meningeal disease, the cladribine concentration in the cerebrospinal fluid exceeds that in plasma. Cladribine is a prodrug and needs intracellular phosphorylation to active nucelotides. The intracellular concentration of these metabolites is several hundred-fold higher than that of cladribine in plasma and they are retained in leukaemia cells with half-lives between 9 and > 30 hours depending on diagnosis and sampling schedule. There is no correlation between the plasma concentration of cladribine and that of the intracellular metabolites. The renal clearance of cladribine is 51% of total clearance and 21 to 35% of an intravenously administered dose is excreted unchanged in the urine. Pretreatment with cladribine increases the intracellular accumulation of the active metabolite of cytarabine, cytosine arabinoside 5'-triphosphate, by 36 to 40%.