The frequency of chronic renal insufficiency among cancer patients is unclear. The aim of this study was to determine the frequency of impaired renal function within a population of cancer patients. One thousand two hundred seventeen patients (563 women, 654 men) with cancer underwent serum creatinine concentration and glomerular filtration rate (GFR) evaluations. The Cockcroft-Gault formula was used to estimate the GFR from the creatinine clearance (Cl-cr). Renal insufficiency was defined as a GFR <= 90 mL/min. Among this population, 72 (5.9%) demonstrated an abnormal serum creatinine concentration (>1.2 mg/dL). According to the Cockcroft-Gault formula evaluations, however, 330 (27.1%) of the patients had an estimated GFR <90 mL/min. Among these, the Cl-cr was between 60 and 89 mL/min in 241 patients (19.8%); 30 and 59 mL/min in 75 patients (6.2%); and 15 and 29 mL/min in 7 patients (0.6%); 7 patients (6%) had a Cl-cr <15 mL/min. As a result, 21.2% of patients demonstrating a normal serum creatinine level had abnormal renal function. Renal function should be evaluated in all cancer patients, regardless of their serum creatinine level, before any drug regimen is administered. The Cockcroft-Gault formula appears to be more accurate than serum creatinine concentration for diagnosing renal insufficiency in patients with cancer, but more prospective studies in this population will be necessary to confirm this finding.