Fludarabine monophosphate is a new antimetabolite with demonstrated activity in chronic lymphocytic leukemia (CLL). We have investigated the practicality of utilizing fludarabine in combination with chlorambucil in a disease-specific phase I trial. Twenty-one patients with advanced and previously treated, relapsed or refractory CLL were treated with chlorambucil plus fludarabine. Chlorambucil was given day 1 at 15 or 20 mg/m2 per os and fludarabine days 1-5 at 10, 15, or 20 mg/m2 intravenously, every 28 days. We concluded that with chlorambucil 15 mg/m2, the maximum tolerated dose for fludarabine was 20 mg/m2 in this patient population with this scheduling. Dose-limiting toxicity was thrombocytopenia. A low incidence of peripheral neuropathy, rash, pulmonary fungal infection, and acute tumor lysis syndrome was also encountered. Although responses were observed, it was impossible from this study to determine whether the combination was better than fludarabine alone in this heavily pretreated population. This study does, however, demonstrate the feasibility of exploring the utility of such a combination in previously untreated patients. An intergroup phase III trial utilizing this combination has been initiated.