OBJECTIVE The purpose of this study was to identify factors that predict successful work hardening outcomes. Two measures of success were used: return to work and case closure (ie, resolution of medical treatment issues). METHODS Persons with spine-related injuries who completed a work hardening program were the subjects. One hundred fifteen subjects participated in a 3-month follow-up survey, and 111 subjects participated in a 12-month follow-up survey. METHODS Data were collected on subject characteristics, treatment history, job factors, program payer, and program factors. Subjects were contacted by telephone at 3 and 12 months after program completion to determine work status. Logit analysis was used to identify predictors of successful versus unsuccessful outcomes. RESULTS Three months after program completion, 68% of the subjects had returned to work and 86% had successful case closure. Twelve months after program completion, 77% of the subjects had returned to work and 90% had successful case closure. The more treatment subjects received prior to entering the program, the less likely they were to be working or achieving case closure following treatment. Subjects' work status and initial time off of work were factors predicting early return to work, but not 12 months after program completion. Subjects who were working with an attorney were less likely to achieve case closure than those who were not working with an attorney. Subjects who were satisfied with the program were more likely to have achieved case closure or return to work than those who were not satisfied with the program. CONCLUSIONS Several factors have been identified that predict successful work hardening outcomes. This information can be used to identify clients who are unlikely to benefit from work hardening.