It is well known that women with a history of manic-depressive or puerperal affective psychosis are at particularly high risk of relapse in the puerperium. This paper describes the use of lithium given during or after pregnancy to women with a history of bipolar illness or puerperal affective psychosis. The rate of puerperal relapse in these subjects was compared with that in a similar group of women not on lithium. The significantly better outcome of the treatment group highlights the need for a prospective controlled trial looking at the effectiveness of lithium in minimising puerperal bipolar relapse.