PURPOSE: Fludarabine is a renally excreted agent that is an effective treatment for chronic lymphocytic leukemia (CLL), a disease predominantly of the elderly. We sought to determine whether age, renal function or pretreatment hematologic status predicted toxicity of fludarabine treatment for CLL. METHODS: We evaluated 192 patients with previously untreated B-cell CLL who were entered onto the fludarabine treatment arm (25 mg/m(2) daily for 5 days every 28 days) of CALGB study 9011, an intergroup study with participation from SWOG, CTG/NCI-C and ECOG. Patients were required to have serum creatinine within 1.5 times normal. Hematologic indices and infections were recorded during the first 28-day cycle of treatment. A time-to-toxicity endpoint was evaluated over the entire course of fludarabine treatment. Creatinine clearance (CrCl(est)) was estimated using serum creatinine, age and body mass index. RESULTS: The median age was 64 years (range 37-87 years) and median CrCl(est) was 62 ml/min (range 27-162 ml/min, interquartile range 52-79 ml/min). We found no association between age and incidence of hematologic toxicity or infection during the first cycle of treatment. There was a strong association between CrCl(est) and the time-to-toxicity endpoint. Patients with CrCl(est) below 80 ml/min had increased incidence of toxicity during their treatment course ( P<0.0001). Pretreatment anemia, thrombocytopenia and Rai stage were highly associated with the incidence of neutrophil toxicity and grade III/IV hematologic toxicities during the first cycle of treatment ( P<0.0001). CONCLUSIONS: Patient age was not an independent risk factor for fludarabine-related toxicity, but CrCl(est) was associated with time to toxicity.