BACKGROUND: Despite continuing political, legal and moral debate on the subject, assisted suicide is permitted in only a few countries worldwide. However, few studies have examined the impact that witnessing assisted suicide has on the mental health of family members or close friends. METHODS: A cross-sectional survey of 85 family members or close friends who were present at an assisted suicide was conducted in December 2007. Full or partial Post-Traumatic Distress Disorder (PTSD; Impact of Event Scale-Revised), depression and anxiety symptoms (Brief Symptom Inventory) and complicated grief (Inventory of Complicated Grief) were assessed at 14 to 24 months post-loss. RESULTS: Of the 85 participants, 13% met the criteria for full PTSD (cut-off≥35), 6.5% met the criteria for subthreshold PTSD (cut-off≥25), and 4.9% met the criteria for complicated grief. The prevalence of depression was 16%; the prevalence of anxiety was 6%. CONCLUSION: A higher prevalence of PTSD and depression was found in the present sample than has been reported for the Swiss population in general. However, the prevalence of complicated grief in the sample was comparable to that reported for the general Swiss population. Therefore, although there seemed to be no complications in the grief process, about 20% of respondents experienced full or subthreshold PTSD related to the loss of a close person through assisted suicide.