Background: Albumin for intravenous infusion is marketed in two concentrations, 20% and 5%, but how they compare with regard to plasma volume expansion over time is unclear.Methods: In a prospective crossover study, 12 volunteers received 3 ml kg À1 of 20% albumin and, on another occasion, 12 ml kg À1 of 5% albumin over 30 min. Hence, equivalent amounts of albumin were given. Blood was collected on 15 occasions over 6 h. Mass balance and volume kinetics were used to estimate the plasma volume