Evaluating balance can be an important part of the rehabilitation protocol of an athletic injury. One purpose of this study was to determine the intertester and intratester reliability scores of single-leg stability on a platform of gradually decreasing stability using the Biodex Stability System (BSS). The second purpose was to determine intertester and intratester reliability scores of subject foot placement on the BSS. Subjects (N = 19) underwent a familiarization session on Day 1 that included five 30-s balance tests on the BSS. In each of the five tests, platform stability gradually decreased over the 30 s. Subjects were tested without footwear at all times. On the second day, each subject was tested twice by the same investigator and once by a second investigator using the same 30-s test. Investigator tests were counterbalanced to eliminate order effects. Intertester intraclass correlations (ICCs) ranged from .70 to .42 for stability index and from .93 to .54 for foot placement. Intratester ICCs ranged from .82 to .43 for stability index and from .81 to .55 for foot placement. The overall stability index scores were the most reliable stability scores (.82 for intratester and .70 for intertester). A 30-s, single-leg, gradually decreasing platform stability test appears to be highly reliable when performed on the BSS.