In this paper, several newly developed techniques for the estimation of the fluid status in hemodialysis patients were reviewed. Whereas echography of the inferior caval vein and the measurement of ANP and cGMP levels merely provide information about the intravascular volume, conductivity measurements are able to detect changes in the extracellular and intracellular compartments without being able to differentiate between the intravascular and interstitial fluid spaces. Echography of the inferior caval vein as a tool to assess over- and underhydration has been successfully validated against objective standards as right atrial pressure, total blood volume and the change in hemodynamic parameters during dialysis. Conductivity measurements were significantly related to vena cava measurements before and after dialysis. Whereas ANP levels were significantly related to the vena cava diameter before dialysis, in another group of patients, only a significant relation between the vena cava diameter and cGMP was observed in patients with normal left atrial hemodynamics, whereas they were not in patients with a dilated left atrium. Furthermore, in normovolemic patients with mitral insufficiency, ANP levels after dialysis remained increased compared to patients without mitral insufficiency, suggesting that, in addition to volume expansion, also altered left atrial hemodynamics influence the release of cGMP and ANP. Conductivity measurements and ANP before and after dialysis were not related, whereas only cGMP after dialysis was significantly related to conductivity measurements. ANP and cGMP were not related to the change in hemodynamic parameters during dialysis, questioning their reliability in the assessment of underhydration.