The immune responsiveness of women is altered during pregnancy in order to retain protective properties against disease and at the same time allow tolerance of the fetus. Diseases such as pre-eclampsia (PE) have been suggested to arise as a result of maladaptations in these immune alterations. Here we evaluate the effect of PE on the composition of peripheral blood lymphocyte subpopulations using lymphocyte surface antigen expression. Fifty-four women of various parities with pregnancy-induced hypertension (PIH) (39 non-proteinuric and 14 proteinuric) and matched controls (30 normotensive pregnant women (NTP) and 15 healthy non-pregnant women (NP)) were investigated. Monoclonal antibodies specific for human T lymphocytes and subpopulations: CD2, CD3, CD4, CD8, CD19 and activation markers: CD25, CD45RA, CD45RO, CD54 AND HLA(-)DR were used and detected using a two-colour fluorescence analysis with an automated flow cytometer. The total number of T lymphocytes: CD2, CD3, CD4, CD8 and CD19 were significantly decreased in PIH particularly PE (P<0.05). T cells expressing NK surface markers (CD3/CD16(+)CD56) and CD4 cells expressing HLA(-)DR were higher in PE. CD8(+)HLA(-)DR(+) cells and T-helper cells expressing adhesion molecules) CD4(+)CD54(+)) were higher in NTP than in NP and PE (P<0.05, 0.05). PE is associated with elevated levels of CD4(+)HLA(-)DR(+), and CD3(+)NK cells but decreased total numbers of T lymphocytes, and the CD3(+)CD25(+) subpopulation. These findings indicate systemic alterations in maternal immunity associated with the PE state. This feature of the disease may contribute to abnormal adaptation to pregnancy resulting in PE and PIH, promoting adverse outcomes including pregnancy loss.