PURPOSE To quantify the incidence and severity of tumor lysis syndrome (TLS) as a consequence of fludarabine therapy in patients with advanced chronic lymphocytic leukemia (CLL). PATIENTS AND METHODS A retrospective review and questionnaire follow-up of clinical and laboratory data were performed on patients with intermediate or high-risk CLL on the National Cancer Institute Group C protocol or special exception mechanisms, or phase II trials of fludarabine, for whom adverse drug reports of TLS were available. Fludarabine was administered at a dose of 20 to 40 mg/m2 per day for 5 days at monthly intervals. RESULTS Among the 6,137 patients, TLS was suspected in 26 (0.42%), with clinical and laboratory features consistent with TLS present in 20 (0.33%). Prophylaxis against TLS had been administered to 60% of these patients. Clinical or laboratory features were similar to patients who did not develop TLS. Of the patients with TLS, 90% had high-risk CLL, 60 months of prior disease duration, with a median pretreatment WBC of 109 x 10(9)/L, two prior regimens, lymphadenopathy in 89%, splenomegaly and/or hepatomegaly in 90%. TLS developed on approximately day 7 and lasted a median of 9.5 days. Dialysis was required in 30% during the TLS episode; 20% of patients died during cycle one of fludarabine therapy with renal failure, and another 20% died of infection or congestive heart failure. Six patients were retreated with fludarabine without recurrent TLS. CONCLUSION TLS after fludarabine therapy is extremely uncommon, but may be associated with significant morbidity and mortality.