Sixteen out of 78 patients (20.5%) on chronic hemodialysis had a mean hematocrit below 0.20 L x L-1 during a six months follow-up period. Three of the 16 had been previously binephrectomized. Eleven of the remaining patients were investigated by 51Cr labelling of red cells. Blood loss in the dialyzer and blood lines was measured at 2.72 +/- 1.90 ml of red blood cells (0.97-7.31). Occult intestinal blood loss was detected in 2 patients. Clinically latent hypersplenism was evidenced in all patients, as demonstrated by an increased splenic-cardiac ratio (99.5 +/- 38% and a decreased red blood cell half-life (16.3 +/- 2.3 days). Hypersplenism and blood loss during dialysis are two underestimated but major contributory factors in the increased anemia observed in chronically hemodialyzed patients.