Personality traits have traditionally been viewed as stable, but recent studies suggest that they could be affected through psychological treatment. Internet-based cognitive behavior therapy (ICBT) for severe health anxiety (DSM-IV hypochondriasis) has been shown to be effective in reducing health anxiety, but its effect on measures of personality traits has not been investigated. The main aim of this study was to investigate the impact of ICBT on personality traits in the three broad dimensions--neuroticism, extraversion and aggression. We hypothesized that participants in ICBT would reduce their level of neuroticism compared to controls that did not receive the active treatment. No specific predictions were made regarding extraversion and aggression. Data from a randomized controlled trial were used in which participants were allocated to 12 weeks of ICBT (n = 40) or to a basic attention control condition (n = 41). Personality traits were assessed with the Swedish Universities Scales of Personality and the primary outcome of health anxiety was the Health Anxiety Inventory. There was a significant interaction effect of group and time on neuroticism-related scales, indicating larger pre- to post-treatment reductions in the Internet-based CBT group compared to the control condition. Analyses at 6-month follow-up showed that changes were stable. Traits relating to extraversion and aggression were largely unchanged. This study is the first to demonstrate that a brief ICBT intervention for severe health anxiety causes long-term changes in measures of personality traits related to neuroticism. The treatment thus has a broader impact than just reducing health anxiety. Clinicaltrials.gov (ID NCT00828152).