Objectives Using a prospective design, the objective of this study was to determine the relationship between workplace bullying and mental distress. Methods Altogether, 1971 Norwegian employees, recruited from 20 organizations, answered questions regarding workplace bullying and mental distress at both baseline and follow-up. Baseline data were gathered between 2004-2006, and follow-up data were gathered between 2006-2009. The time-lag between baseline and follow-up was approximately two years

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... or all the respondents in all the organizations. The factors measured in the study were individual characteristics, mental distress measured with the Hopkins Symptom Checklist (HSCL-10), self-reported workplace bullying measured with a single item from the General Nordic Questionnaire for Psychological and Social Factors at Work (QPSNordic) and job demands and job control assessed by QPSNordic. Results A hierarchical multiple linear regression analysis adjusted for mental distress, sex, age, job demands, and job control at baseline [β=0.05, 95% confidence interval (95% CI) 0.03-0.17] and a repeated measures ANOVA adjusted for sex and age [F(3,1965)=38.37; partial η 2 =0.06] showed that workplace bullying predicted mental distress. Furthermore, a multiple binary logistic regression analysis adjusted for bullying, sex, age, job demands, and job control at baseline [odds ratio (OR) 2.30, 95% CI 1.43-3.69] showed that mental distress was a predictor of bullying. Conclusions We found support for the notion that self-reported workplace bullying is a predictor of mental distress two years later. Bullying had an independent effect on mental distress after adjusting for job demands and job control. Mental distress was also found to be a predictor of bullying, indicating that the reverse relationship is also important.