Targeting aberrant DNA hypermethylation in chronic lymphocytic leukaemia (CLL) and non-Hodgkin lymphoma (NHL) with decitabine may reverse epigenetic silencing in B-cell malignancies. Twenty patients were enrolled in two phase I trials to determine the minimum effective pharmacological dose of decitabine in patients with relapsed/refractory CLL (n = 16) and NHL (n = 4). Patients received 1-3 cycles of decitabine. Dose-limiting toxicity (DLT) was observed in 2 of 4 CLL and 2 of 2 NHL patients receiving decitabine at 15 mg/m(2) per d days 1-10, consisting of grade 3-4 thrombocytopenia and hyperbilirubinaemia. Six patients with CLL received decitabine at 10 mg/m(2) per d days 1-10 without DLT; however, re-expression of methylated genes or changes in global DNA methylation were not observed. Therefore, a 5-day decitabine schedule was examined. With 15 mg/m(2) per d decitabine days 1-5, DLT occurred in 2 of 6 CLL and 2 of 2 NHL patients, consisting of grade 3-4 neutropenia, thrombocytopenia, and febrile neutropenia. Eight patients had stable disease. In 17 patients, there were no significant changes in genome-wide methylation or in target gene re-expression. In conclusion, dose-limiting myelosuppression and infectious complications prevented dose escalation of decitabine to levels associated with changes in global methylation or gene re-expression in CLL and NHL.