A fluid replacement system comprising: a subsystem for determining an amount of fluid infused into a patient from a first fluid source; and a patient's urine output; a pump configured to pump fluid from the first fluid source into the patient; a user interface including means for setting a desired positive, negative, or zero fluid balance for the patient; and means for setting an amount of at least one additional fluid delivered to the patient by other means than the pump of the system; and a processing subsystem responsive to the set desired positive, negative, or zero fluid balance, the set amount of at least one additional fluid delivered to the patient; and the subsystem said processing subsystem being configured to: determine the patient's urine output and the amount of fluid infused into the patient from the first fluid source, calculate an actual fluid balance based on the determined urine output, the determined amount of fluid infused into the patient from the first fluid source, and the set amount of at least one additional fluid delivered to the patient; and to control the pump based on the calculated actual fluid balance and a set desired positive, negative or zero fluid balance so that the actual fluid balance becomes the same as or approximately the same as the desired positive, negative or zero fluid balance. a subsystem for determining an amount of fluid infused into a patient from a first fluid source; and a patient's urine output; an amount of fluid infused into a patient from a first fluid source; and a patient's urine output; a pump configured to pump fluid from the first fluid source into the patient; a user interface including means for setting a desired positive, negative, or zero fluid balance for the patient; and means for setting an amount of at least one additional fluid delivered to the patient by other means than the pump of the system; and means for setting a desired positive, negative, or zero fluid balance for the patient; and means for setting an amount of at least one additional fluid delivered to the patient by other means than the pump of the system; and a processing subsystem responsive to the set desired positive, negative, or zero fluid balance, the set amount of at least one additional fluid delivered to the patient; and the subsystem the set desired positive, negative, or zero fluid balance, the set amount of at least one additional fluid delivered to the patient; and the subsystem said processing subsystem being configured to: determine the patient's urine output and the amount of fluid infused into the patient from the first fluid source, calculate an actual fluid balance based on the determined urine output, the determined amount of fluid infused into the patient from the first fluid source, and the set amount of at least one additional fluid delivered to the patient; and to control the pump based on the calculated actual fluid balance and a set desired positive, negative or zero fluid balance so that the actual fluid balance becomes the same as or approximately the same as the desired positive, negative or zero fluid balance. determine the patient's urine output and the amount of fluid infused into the patient from the first fluid source, calculate an actual fluid balance based on the determined urine output, the determined amount of fluid infused into the patient from the first fluid source, and the set amount of at least one additional fluid delivered to the patient; and to control the pump based on the calculated actual fluid balance and a set desired positive, negative or zero fluid balance so that the actual fluid balance becomes the same as or approximately the same as the desired positive, negative or zero fluid balance. the determined urine output, the determined amount of fluid infused into the patient from the first fluid source, and the set amount of at least one additional fluid delivered to the patient; and to control the pump based on the calculated actual fluid balance and a set desired positive, negative or zero fluid balance so that the actual fluid balance becomes the same as or approximately the same as the desired positive, negative or zero fluid balance.