Prison suicide rates are increasing. The impact of witnessing a suicide or how many people do so is unknown. The aim of this study was to find how many young people detained in a Young Offenders' Institution (YOI) have had contact with another's suicide attempt and to test for association between this and own self-harming behaviour. A questionnaire, developed for the project, was distributed within a YOI. 74% (355/480) responded. One hundred and fifty-one (43%) of the young men reported that they knew someone who had attempted suicide, 48 of them knowing someone who had thereby died. The contact with a non-fatal suicide attempted by others was associated with own self-harm (OR 1.73, CI 1.39-21.4, p less than 0.01); contact with completed suicide had no such association. The most important factor that increased the chance of knowing a suicide attempter was actual length of time spent in prison up to the time of the study; 95 (63%) of the young men with such contact had it in prison. No other significant vulnerability factors were identified. It seems likely that imprisonment is a specific vulnerability factor for self-harm, one explanation being that it increases the risk of exposure to a contact with suicidal behaviour by others. Further work is called for to test relationships between such exposure and own behaviour over time, and to gather first hand accounts from the young men in such circumstances in order to understand the meaning for them of the contact with suicidal behaviour by others. This would inform appropriate intervention strategies, and could save much morbidity, and even mortality.