This study investigated predictors of outcome for a three-month group-exercise program among a community-dwelling Japanese population aged over 65 (n=42). The primary outcome variable was health-related quality of life (HRQOL) using the Short Form 36 (SF-36). Candidate predictors included demographic variables, medical history of chronic diseases, and lifestyle-related factors, including self-reported home exercise at baseline examination. At follow-up, only the vitality domain of the SF-36 was significantly improved. Self-reported home exercise and younger age at baseline were significant predictors of a good outcome in the mental and physical components, respectively, of the SF-36. These results indicate that group exercise may be effective to improve the vitality component of HRQOL for Japanese seniors. However, the effects on the physical components of HRQOL are limited. Home exercise and younger age may influence the effects of group exercise on HRQOL for this population.