A 35 years old male is presented. Eight months after receiving a renal allograft his hypertension worsened and his packed red cell volume raised to 53.7%. Enalapril was started and a 500 ml phlebotomy was performed. In three occasions packed red cell volumes decreased excessively and enalapril was discontinued. Finally, the drug was replaced with losartan, normalizing blood pressure values and stabilizing packed red cell volumes. This case illustrates the different reductions in packed red cell volumes induced by enalapril and losartan. These differences could have a therapeutic relevance.