There is good evidence from recent studies that depression is familial, and that a substantial proportion of the variation in liability is explained by genes. Suicidal behavior, including completed suicide, also seems to cluster in families. First-degree relatives of individuals who have committed suicide (included dizygotic twins) have more than twice the risk of the general population. For identical co-twins of suicides, the relative risk increases to about 11. Applying a simple structural equation model to the published data suggests a heritability for completed suicide of about 43% (95% confidence intervals 25-60). 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 committing suicide have a diagnosis of depression, it seems probable that the overlap is incomplete. The mode of inheritance of suicidal behavior is almost certain to be complex, involving many genes. There have already been some initial studies of allelic association with polymorphisms in candidate genes such as those involved in serotonergic transmission. Further progress is likely to come from candidate gene and linkage disequilibrium studies that are capable of detecting multiple genes of small effect.