The relationship between Hemoglobin S (Hb S) level and simultaneous values of reticulocyte count, hemoglobin (Hb), and hematocrit (Hct) were studied in six patients with sickle cell disease who were receiving a transfusion protocol because of cerebrovascular accidents. There was good correlation between Hb S and reticulocyte count (r = 0.601), Hb (r = -0.530), and Hct (r = 0.479). Although three of the six patients had similar patterns of relationship of Hb S to reticulocyte count and four of six had similar patterns of relationship of Hb S to Hb and Hct, the other patients had distinctly different patterns. "Decision levels" of reticulocyte count, Hb and Hct, indicating the probable need for transfusion, were determined by the point at which the regression line plus 1 standard error would yield the maximum Hb S considered safe. Although all individual decision levels could separate patients with regard to the need for transfusion, decision levels for the group as a whole could not do so for Hb or Hct and could do so for reticulocyte count only with reduced specificity and predictive value. After an individual patient's pattern of relationship has been established, estimates of Hb S based on reticulocyte count, Hb, or Hct can be used as an inexpensive screen for the need for transfusion.