The outcome of 584 AID pregnancies was analysed in order to examine the possible role of immune mechanisms in the development of pre-eclampsia (PE). The overall incidence of PE was high (9.3%) compared with the expected incidence of 0.5-5.0%. The incidence was similar in both multigravid and primigravid women. The expected protective effect of a previous pregnancy was not seen, with a 47-fold increase in PE (observed versus expected) in AID pregnancies after a previous full-term pregnancy and a 15-fold increase after a previous pregnancy of short duration. In the context of AID pregnancies these findings tend to support the proposed 'fresh mating' concept of PE although the incidence is lower in multigravidas after a full-term (4.7%) than after a partial pregnancy (15.7%). This suggests that it may only be full-term pregnancy which protects against the development of PE. THe category and duration of infertility and the duration of marriage did not affect the incidence of PE except where the male partner was oligozoospermic. In this case the incidence of PE was higher where infertility was 3 years or longer. Further analysis of the findings failed to provide clear evidence to discriminate the role of HLA and tissue-specific non-HLA antigens in the feto-maternal relationship as determinants of the development of PE. Neither did the results support the operation of immunological enhancement induced by prior coital antigen exposure in the genesis of PE, but raised instead the possibility of the operation of a mechanism of sensitisation of the female partner to seminal components over a long period of time when the husband was oligozoospermic compared with one whose husband was azoospermic.