The pharmacokinetics of antiretroviral drugs in pregnancy is poorly understood. We reviewed the use of therapeutic drug monitoring (TDM) in clinical settings to document plasma concentrations of lopinavir during pregnancy and investigated how clinicians acted upon TDM results. A retrospective review was carried out of all HIV-infected pregnant women taking boosted lopinavir-based highly active antiretroviral therapy (HAART) at five National Health Service (NHS) centres in the UK between May 2004 and March 2007. Seventy-three women in receipt of lopinavir were identified, of whom 89% had plasma lopinavir concentrations above the suggested minimum recommended for wild-type HIV. Initial TDM results prompted dosage change in 10% and assessment of adherence and/or pharmacist review in 11%. TDM was repeated in 29%. TDM can play an important role in the clinical management of HIV-positive pregnant women, allowing informed dose modification and an alternative measure of adherence.