It is hypothesized that significantly more living monozygotic than dizygotic co-twins of twin suicide victims would themselves have attempted suicide. The absence or presence of a suicide attempt at any time was investigated among 26 living monozygotic co-twins & 9 living dizygotic co-twins of twin suicide victims. 10 of 26 surviving monozygotic co-twins had attempted suicide; none of the dizygotic co-twins had. Results support previous findings suggesting a genetic role in suicidal behaviours.