Changes involving the immune system may be instigators of, or consequent upon, disorders in other systems in the natural history of gestosis. The extent of the immune 'disorder' may not correlate with the severity of the disease as manifest in either eclampsia or fetal death. A number of variables, e.g. hormonal and genetic, could affect any immunological trigger mechanism or any secondary altered immune response. Thus attempts to find an immunological factor in gestosis that operates in all cases may prove unrewarding. Certain matings, immunogenetic constitutions, or certain feto-maternal immune relationships may predispose to a particular pathology and directly or indirectly potentiate other pathological processes. Others may either have no effect or conceivably have even a homeostatic effect protecting the maternal host from pathological change. The picture of the role of immunological factors in the pathogenesis of pregnancy hypertension is no more, as yet, than a few hesitant, unrelated lines. However, the lines are getting more numerous and stronger.