The objective of this study was to assess the interinstrument reliability of six RT3 accelerometers for measuring physical activities. Each of the six healthy participants, mean age 36.1 years (SD 9.4), carried six RT3 accelerometers (same type and same producer) simultaneously placed ventrally at the waist belt. The participants performed three standardized activities: walking on a treadmill at 3.0 km/h and 5.0 km/h, and sitting on a chair. Each activity lasted 5 min. The recordings of the accelerometers were compared with each other to assess interinstrument reliability. A correlation of 0.75 or higher was interpreted as sufficient. The mean Pearson correlation between the six accelerometers was r = 0.78 (0.46-0.97). The intraclass correlation between the accelerometers was 0.75 (95% confidence interval: 0.46-0.95, P less than 0.01). In conclusion, the interinstrument reliability of the RT3 accelerometer is sufficient. However, the lower limit of the confidence interval is low, indicating a challenge to the reliability.