Purpose: Breast cancer is the most common cancer of Finnish women. Peer support could be a way to help breast cancer patients to deal with the disease but studies on its effectiveness have produced conflicting results. The aim of this randomized controlled trial was to study the effectiveness of peer support on health-related quality of life (HRQoL) of breast cancer patients. Methods: Patients with recently diagnosed breast cancer at the Helsinki University Hospital were randomly allocated to intervention (n = 130) or control (n = 130) groups. The intervention group patients received peer support via telephone one to five times according to their preference. The control group received usual care only. HRQoL was assessed with generic (15D) and disease-specific (EORTC QLQ-30 and its breast cancer specific module BR23) instruments at baseline and at 3-, 6-, and 12-month follow-up points. Results: The mean (SD) age of the patients was 60.0 (10.5) years and their baseline mean 15D score 0.922 (0.066). At baseline, the intervention and control groups did not differ from each other. During follow-up, the 15D score deteriorated statistically significantly (p < 0.001) and clinically importantly in both groups but slightly less in the intervention group although the difference was not significant. Regarding individual 15D dimensions, the EORTC-QLQ30, or its breast-specific module, peer support did not show any consistent advantage compared to usual care. Conclusion: Peer support had no clear effect on the HRQoL of breast cancer patients.