AIMS: Little has been reported on self-harm among the UK Armed Forces, partly due to the difficulties in recording self-harm, within an often-difficult-to-reach population. This study assesses the lifetime prevalence of attempted suicide and self-harm within currently serving and ex-service personnel of the UK Armed Forces. METHODS: Telephone interviews were conducted with 821 personnel who had previously participated in the King's Centre for Military Health Research military health study. Within the telephone interview, participants were asked about attempted suicide and episodes of self-harm. RESULTS: A lifetime prevalence of 5.6% for intentional self-harm (self-harm or attempted suicide) was reported. Intentional self-harm was associated with psychological morbidity (in particular, post-traumatic stress disorder) and adverse experiences in childhood. Ex-service personnel reported lifetime prevalence more than double that of serving personnel (10.5% vs 4.2%, respectively). Participants reporting intentional self-harm were younger (34.4 years vs 39.8 years). CONCLUSION: A lifetime prevalence of 5.6% for attempted suicide and self-harm is higher than previous research has suggested. Younger service personnel, those who have experienced adversity in childhood, those with other psychological morbidity, and ex-service personnel are more likely to report self-harm behaviours.