Ahstract--A Phase I study with 5-aza-2'-deoxycytidine (5-AZA-CdR) was performed on children with acute leukemia who were resistant to conventional chemotherapy. The objective of this study was to find the dose-schedule of 5-AZA-CdR that produced a potent antileukemic effect and to evaluate its toxicity. At doses of 0.75-30 mg/kg administered as 12-30-h i.v. infusion and 10 mg/kg administered as i.v. bolus the antileukemic effect produced by 5-AZA-CdR was of short duration. A more potent antileukemic effect was observed when the dose of 5-AZA-CdR was increased from 36 to 80 mg/kg administered as 36--44-h i.v. infusion. At this dose range two of nine patients obtained an M~ marrow and one of nine patients obtained an M 2 marrow. Clearing of meningeal leukemia occurred in two patients. Using an in vitro test which measures the inhibition of DNA synthesis in blood and marrow leukemic cells by cytosine arabinoside, five of nine patients showed signs of drug resistance after 5-AZA-CdR treatment at the higher doses. A bioassay which quantitates the growth inhibitory effects of 5-AZA-CdR on LI210 leukemic cells was used to estimate the concentration of this agent in the body fluids. At an infusion rate of 1.0 mg/kg/h the steady state plasma concentration was estimated to be 0.5 ~g/ml. The plasma half-life of 5-AZA-CdR was 12 rain. In two patients inhibitory concentrations of 5-AZA-CdR were detected in the cerebral spinal fluid.