The achievement of a normal hydration state is one of the major targets of hemodialysis. It is based on the estimation of ''dry weight'', which corresponds to the normal body fluid content. Even though the concept of dry weight in hemodialysis patients is clinically undisputed, it is not always easy to achieve in this population. Assessment of the hydration state by clinical examination is imprecise and often unreliable. Measurement of the inferior vena cava (IVC) diameter has been shown to reflect individual fluid status. The relationship between the variation of the IVC diameter before and after the hemodialysis session and weight loss has been investigated. Measurement of the IVC diameter by ultrasound is considered a valid measure of the hydration state and can be routinely used in hemodialysis patients. Moreover, a relationship between IVC diameter, respiratory activity and the hydration state, evaluated by considering both plasma volume and central venous pressure, has been demonstrated. In conclusion, assessment of the hydration state based on blood pressure and central venous pressure can be considered reliable only in patients without signs of heart failure.