The Hoffmann reflex (H-reflex) is a measure of motoneuron pool excitability, which is valuable in determining muscle inhibition caused by joint damage (arthrogenic muscle inhibition). In order to detect changes in H-reflex due to injury, the reliability of such a measurement must be established. The purpose of this study was to establish the intrasession and intersession reliability of soleus H-reflex in a supine and standing position. Thirteen healthy volunteers (age 10 +/- 2.63 yr, height 171.35 +/- 10.19 cm, mass 69.62 +/- 13.03 Kg) with no lower extremity orthopedic or neurological disorders within the past year participated in this study. To determine the intrasession and intersession reliability of this measure in a supine resting position and a one-leg standing position, EMG data were collected from the soleus while the tibial nerve was stimulated in the popliteal space. A high voltage (120-200 V), short duration (1.0 msec) stimulus was automatically triggered, eliciting a reflex twitch detected by surface EMG. Several of these measurements were performed with 20 second rest intervals to find the maximum H-reflex. The maximum H-reflex was located by adjusting the intensity of the stimulus. Once a maximum H-reflex was found, 12 measurements were taken in that position with 20 second rest intervals. These steps were repeated for each position (supine and standing) at the same time for 5 consecutive days. Intrasession reliability was computed using 12 measurement trials (12), 12 measurement trials dropping the high and low score (12x), the first 7 measurement trials dropping the high and low score (7x), and the first 5 measurement trials (5). Intrasession and intersession reliability over five consecutive days was estimated using intraclass correlation coefficients (ICC (3, 1)). The supine intrasession reliability measurements were as follows: 0.932 (12), 0.932 (12x), 0.935 (7x), and 0.932 (5). The standing intrasession reliability was 0.853 (12), 0.852 (12x), 0.865 (7x), and 0.862 (5). The intersession reliability was 0.938 in the supine position and 0.803 in the standing position. These results indicate that the H-reflex measured using our protocol in a supine and standing position is a reliable assessment within sessions and between sessions. Five measurements are sufficient to observe reliable measurements within a single session. Most importantly, this data shows that the H-reflex is a reliable assessment that may be used to measure small changes in motoneuron pool excitability over time.