Thirty-three patients with chronic lymphocytic leukemia (CLL) with advanced Rai stage (III-IV) or progressive Rai stage (0-II) disease were treated with fludarabine as a single agent. Eleven patients (33%) obtained a complete remission (CR), 13 (39%) a clinical CR with residual nodules as the only evidence of disease (nodular partial remission [PR]), and two patients (6%) achieved a PR for a total response rate of 79%. Response was rapid, usually occurring after three to six courses of treatment. The major morbidity was infection. Febrile episodes occurred in 13% of the courses (pneumonia 6%, minor infection 4%, and transient fever of undocumented cause 3%). Fludarabine appears to be the most cytoreductive single agent so far studied in CLL.