The authors analyzed data from 106 adults with chronic renal failure [(CRF), serum creatinine (Ser) > 132.6 μmol/1 (1.5 mg/dl), to define the precision of, and effect of covariables on, the relationship between anemia and CRF. We excluded dialyzed or patients who had received transplants and those with obvious potential nonrenal causes of anemia. Hct was a linear function of BUN and Scr (r = −0.48, and r= −0.67, respectively), and a curvilinear function of creatinine clearance (r =0.68), all p < .0001. Confidence interval analysis revealed a±10 volume percent variation in Hct at any level of renal function. Sex was the only covariable that affected Hct Independently of CRF. This study suggests that CRF may have a different effect on Hct in males and females and indicates 4that the range of Hct possible at a given level of CRF precludes attributing anemia to CRF with assurance.