During high-intensity exercise, intracellular creatine phosphate (PCr) is rapidly broken down to maintain adenosine triphosphate turnover. This has lead to the widespread use of creatine monohydrate as a nutritional ergogenic aid. However, the increase in intracellular PCr and the concomitant increase in intracellular water have not been investigated with regard to their effect on active range of movement (ROM). Forty male subjects (age, 24+/-3.2 years) underwent restricted randomization into 2 equal groups, either an intervention group (CS) or a control group (C). The CS group ingested 25 g.day(-1) of creatine monohydrate for 5 days, followed by 5 g.day(-1) for a further 3 days. Before (24 h before starting supplementation (PRE) and after (on the 8th day of supplementation (POST)) this loading phase, both groups underwent goniometry measurement of the shoulder, elbow, hip, and ankle. Data indicated significant reductions in active ROM in 3 movements: shoulder extension (57+/-11.3 degrees PRE vs. 48+/-11.2 degrees POST, p<0.01), shoulder abduction (183.4+/-6.8 degrees PRE vs. 180.3+/-5.1 degrees POST, p<0.05), and ankle dorsiflexion (14.2+/-4.7 degrees PRE vs. 12.1+/-6.4 degrees POST, p<0.01). There was also a significant increase in body mass for the CS group (83.6+/-6.2 kg vs. 85.2+/-6.3 kg, p<0.05). The results suggest that short-term supplementation with creatine monohydrate reduces the active ROM of shoulder extension and abduction and of ankle dorsiflexion. Although the mechanism for this is not fully understood, it may be related to the asymmetrical distribution of muscle mass around those joints.