Variations in blood concentrations of antiphospholipid antibodies (APA) were investigated through the course of pregnancy in women who had a history of recurrent pregnancy loss, and were related to changes in pregnancy outcome. Serial measurements of APA were made in 16 women with antiphospholipid syndrome (APS) and 16 with negative APA tests pre-pregnancy. There was considerable intraindividual variation in test results through pregnancy. There was a significantly higher ratio of dilute Russell's viper venom time and IgG ACA titre in the first trimester compared with results pre-pregnancy in women with APS. Furthermore, transiently positive APA results were noted in the control group during pregnancy and some women with antiphospholipid syndrome tested negative for APA in mid- and late pregnancy. We have demonstrated clinically important variations in the results of tests for APA during pregnancy in women with APS.