BACKGROUND: The ability of the Sensory Organization Test (SOT) to detect subtle balance problems has been challenged. The Head-Shake Sensory Organization Test (HS-SOT) has been developed to improve the delineation of balance performance. OBJECTIVE: The purposes of this study were: (1) to examine age-related differences in balance measured with the HS-SOT and (2) to establish the test-retest reliability of the HS-SOT in younger adults who were healthy and older adults who were healthy. DESIGN: A test-retest design was used in this observational measurement study. METHODS: Ninety-two younger adults who were healthy (mean age=28.3 years) and 73 older adults who were healthy (mean age=60.3 years) underwent the SOT and the HS-SOT. Seventy-seven of them (56 younger adults, 21 older adults) underwent the same assessments 1 to 2 weeks later. RESULTS: The equilibrium scores in HS-SOT conditions 2 (head movements with eyes closed while standing on a firm surface) and 5 (head movements with eyes closed while standing on a sway-referenced surface) were significantly lower than those in tests without dynamic head movements added (SOT conditions 2 and 5). Older adults attained significantly lower scores in both HS-SOT conditions than their younger peers. The test-retest reliability values, reported as intraclass correlation coefficients (ICC [3,2]), of the HS-SOT scores in conditions 2 and 5 for the younger adults were .85 and .78, respectively; those for the older adults were .64 and .55, respectively. The corresponding minimal detectable change values for the former were 2.7 and 16.2, and those for the latter were 3.6 and 22.7. LIMITATIONS: Only head rotation movements on the horizontal plane were tested. CONCLUSIONS: Adding head movements to the SOT increased the separation of younger adults who were healthy and older adults who were healthy. The HS-SOT has good reliability, and the reported minimal detectable change values may facilitate the interpretation of clinical studies in which the HS-SOT is used to assess changes in balance performance in younger and older adults.