OBJECTIVE: To explore the effect of participatory intervention for workplace improvement on mental health and job performance. METHODS: Eleven assembly lines were randomly allocated to six intervention and five control lines (47 and 50 workers, respectively). The primary outcome was defined as the improvement in the following measures during the intervention period: General Health Questionnaire and WHO Health and Work Performance Questionnaire. RESULTS: General Health Questionnaire scores significantly deteriorated in the control lines, whereas the score remained at the same level in the intervention lines. Health and Work Performance Questionnaire scores increased in the intervention lines, but decreased in the control lines, yielding a significant intervention effect (P = 0.048). CONCLUSION: It is suggested that the participatory intervention for workplace improvement is effective against deterioration in mental health and for improving job performance. © 2009 The American College of Occupational and Environmental Medicine.