OBJECTIVE To identify a correlation between dialysis treatment duration and severity of acquired cystic disease of the end-stage kidney measured by cyst sizes; and assess its effect on spontaneous improvement of anemia. METHODS Ten patients, 6 males and 4 females were selected, who have been on hemodialytic treatment for more than 5 years. There was no patient selected with autosomal dominant polycystic kidney disease. The renal evaluation has been made through ultrasonographic studies. Acquired cystic disease of the end-stage kidney was characterized by finding four or more cysts in both kidneys. The largest cysts were measured for correlation effect. The diagnosis of anemia was established by hematocrit and hemoglobin serum values. It has also been analyzed serum urea, creatinine, albumin, iron, total iron-binding capacity and the per cent saturation of serum transferrin were tested. RESULTS The patients were properly dialysed (serum urea and creatinine = 98.7 +/- 35 mg/dL e 9.7 +/- 2.7 mg/dL, respectively). There were in good nutritional shape (serum albumin = 4.5 +/- 0.5 g/dL) and had normal serum iron level (serum iron = 80 +/- 34 mg/dL). The prevalence of acquired renal cystic disease was 80%. No finding of malignancy has been detected in these cysts. There was a significant correlation between time on dialysis treatment and hematocrit values (R = 0.70; p < 0.05). Cyst sizes had a direct and significant correlation with hematocrit levels (R = 0.74; p < 0.05). CONCLUSIONS These results show that spontaneous improvement on anemia seen in patients on chronic dialysis has a significant correlation with the severity of acquired cystic disease of the end-stage kidney. Our data suggest a functional role of acquired kidney cysts on endogenous erythropoietin production.