Risk for suicide may have heritable contributions. Evidence supporting this hypothesis includes strong and consistent findings from more than 20 controlled family studies indicating nearly 5-fold greater relative risk of suicidal acts among relatives of index cases with suicidal behavior compared to relatives of nonsuicidal controls. Relative risk was greater for completed suicide than for attempts. Contributions of genetic instead of environmental factors are indicated by a higher average concordance for suicidal behavior among co-twins of suicidal identical twins compared to fraternal twins or to relatives of other suicidal subjects, in at least seven studies. Three studies indicate significantly greater suicidal risk, particularly for completed suicide, among biological versus adoptive relatives of suicidal or mentally ill persons adopted early in life. Molecular genetics studies have searched inconclusively for associations of suicidal behavior with genes mainly for proteins required for central serotonergic neurotransmission. Complex interactions of environmental with heritable risk and protective factors for suicide and psychiatric illnesses or vulnerability traits are suspected, but specific intervening mechanisms remain elusive. Familial or genetic risks for psychiatric factors strongly associated with suicide, such as major affective illnesses and alcohol abuse, as well as impulsive or aggressive traits, have not consistently been separated from suicidal risk itself.