Background/aim: This study aimed to compare washed red blood cell (WRBC) transfusion versus nonwashed RBC (NWRBC) transfusion in terms of posttransfusion potassium levels in dialysis patients on a day when the patient did not receive dialysis. Materials and methods: The patients were randomly assigned into two groups, i.e. those receiving WRBCs (n = 21) and those receiving NWRBCs (n = 17). Both groups received one unit of RBCs. Serum potassium and sodium levels were measured before and at the 1st, 2nd, 3rd, 4th, and 6th hours after transfusion. Results: In the WRBC group, the changes in the serum potassium levels at the 3rd, 4th, and 6th hours after transfusion were significant compared with pretransfusion levels. In the serum potassium levels mean decreases by 0.38 ± 0.57 mEq/L at the 3rd hour (P = 0.006), by 0.32 ± 0.47 mEq/L at the 4th hour (P = 0.005), and by 0.32 ± 0.51 mEq/L at the 6th hour (P = 0.009) after transfusion were significant compared with the pretransfusion levels. Conclusion: Although nonwashed RBC transfusion does not change serum potassium levels, washed RBC transfusion significantly reduces serum potassium levels. Washed RBC transfusion is considered to be safer in hemodialysis patients with hyperkalemia and anemia.