One of the indications for therapeutic drug monitoring (TDM) may be the use of antiretroviral agents during pregnancy. We report on a case in which repeated low plasma levels of nelfinavir were observed. After an initial good virologic response, virologic failure appeared to occur, and dose modifications guided by TDM were performed. Although a causal relationship cannot be proven, viral load became undetectable after our interventions. A healthy uninfected daughter was born.