To evaluate the risk of significant plateletpheresis-induced thrombocytopenia in normal volunteer donors undergoing plateletpheresis less frequently than every 56 days and to help understand factors influencing platelet yield in these donors. pre-and postapheresis platelet counts ( x l O 3 / ~l ) and platelet yields were analyzed from 916 plateletpheresis procedures (Fenwal CS-3000) on 607 donors (405 men, 202 women). The mean preaphresis platclet count was 265 k 59 (SD) (range 155-650) (men 256 55 7170-4441: women 284 k 65 [155-650]; P < .001. f-test). After an average platclet yield of 4.08 X LO", the mean postplatelet count was 185 k 46 (range 72-420) (men 184 f 42 180-3451: women 194 k 52 [72-500]: P < 0.1). The percent decrease in platelet count (mean 29.4 k 13, range -28.1-65.5) had positive correlations with platelet yield (P < .01, r = 0. lo), predonation platelet count ( P < .001. r = 0.29), and age ( P < .01, r = .lo) and a negative correlation with weight (P < ,001, r = 0.18) Both the percent decrease and platelet yield were significantly higher in women than in men (P < .OOl. r-test). and platelet yield was best predicted by preapheresis platelet count in both sexes (r = 0.65). The cffects of plateletpheresis on donor platelet count thus vary widely among donors. Although a decrease of greater than 50% was not a rare event (5.9% of all procedures), in only three instances (0.3%) was the actual postapheresis platelet count below 100 (72, 94. 95). Thus, plateletpheresis in normal donors appears unlikely to produce clinically significant thrombocytopenia immediately after apheresis.