OBJECTIVE: To examine the effects of oral creatine (Cr) monohydrate supplementation on muscle Cr concentration, body mass, and total body water (TBW), extracellular water (ECW), and intracellular water (ICW) volumes. DESIGN AND SETTING: After an overnight fast, urinary Cr and creatinine concentrations, muscle Cr concentration, body mass, TBW, ECW, and ICW were measured, and subjects were randomly assigned to either a Cr or a placebo (P) group. The Cr group ingested 25 g/d of Cr for 7 days (loading phase) and 5 g/d for the remaining 21 days (maintenance phase), whereas the P group ingested a sucrose P using the same protocol. All the measures were reassessed immediately after the loading and maintenance phases. SUBJECTS: Sixteen men (age = 22.8 +/- 3.01 years, height = 179.8 +/- 7.1 cm, body mass = 84.8 +/- 11.2 kg) and 16 women (age = 21.8 +/- 2.51 years, height = 163.4 +/- 5.9 cm, body mass = 63.6 +/- 14.0 kg) involved in resistance training volunteered to participate in this study. MEASUREMENTS: Muscle Cr concentration was determined from the vastus lateralis muscle using a percutaneous needle-biopsy technique. Total body water, ECW, and ICW volumes were assessed using deuterium oxide and sodium bromide dilution analyses. RESULTS: The Cr group experienced a significant increase in muscle Cr concentration, body mass, and TBW. The P group experienced a small but significant increase in TBW only. CONCLUSIONS: The Cr supplementation protocol was effective for increasing muscle Cr concentrations, body mass, and TBW; however, fluid distribution was not changed.