Applying a simple structural equation model to the published data suggests a heritability for completed suicide of about 43%. It is not known at present whether the genes predisposing to suicide are identical with those predisposing to affective disorder, but since only about half of those completing suicide have a diagnosis of depression, is seems probable that the overlap is incomplete. The mode of inheritance of suicidal behaviour is almost certain to be complex, involving many genes. There have already been some initial studies of allelic association with ploymorphism in candidate genes such as those involved in serotonergic transmission. Further progress is likely to come from candidate gene & linkage disequilibrium studies that are capable of detecting multiple genes of small effect. (28 refs.)