MIHIC, S., JR MACDONALD, S. MCKENZIE, and MA TARNOPOLSKY. Acute creatine loading increases fat-free mass but does not affect blood pressure, plasma creatinine, or CK activity in men and women. Med. Sci. Sports Exerc., Vol. 32, No. 2, pp. 291–296, 2000. Creatine monohydrate (CrM) administration may enhance high intensity exercise performance and increase body mass, yet few studies have examined for potential adverse effects, and no studies have directly considered potential gender differences. The purpose of this study was to examine the effect of acute creatine supplementation upon total and lean mass and to determine potential side effects in both men and women. The effect of acute CrM (20 gd 1 5 d) administration upon systolic, diastolic, and mean BP, plasma creatinine, plasma CK activity, and body composition was examined in 15 men and 15 women in a randomized, double-blind experiment. Additionally, ischemic isometric handgrip strength was measured before and after CrM or placebo (PL). CrM did not affect blood pressure, plasma creatinine, estimated creatinine clearance, plasma CK activity, or handgrip strength (P 0.05). In contrast, CrM significantly increased fat-free mass (FFM) and total body mass (P 0.05) as compared with PL, with no changes in body fat. The observed mass changes were greater for men versus women. These findings suggest that acute CrM administration does not affect blood pressure, renal function, or plasma CK activity, but increases FFM. The effect of CrM upon FFM may be greater in men as compared with that in women. Key Words: NUTRITIONAL SUPPLEMENT, ERGOGENIC AID, SIDE EFFECTS, RENAL FUNCTION, CREATININE