Systems and methods for controlling hematocrit during a blood processing procedure. In one aspect, a system includes a first flow path for flowing whole blood from a patient. A separator communicates with the first flow path for separating at least red blood cells from plasma. Second and third flow paths communicate with the separator for respectively flowing the separated plasma and red blood cells from the separator. A flow controller is associated with the second and third flow paths to control fluid communication between the second and third flow paths and addition of separated plasma to the separated red blood cells. The resulting red blood cells and plasma has a resulting hematocrit. Hematocrit is calculated or sensed in operative association with the third flow path and communicated to the flow controller for controlling the amount of plasma added to the third flow path and the resulting hematocrit.