Patients with schizophrenia are eight times more likely than healthy individuals to become infected with the human immunodeficiency virus (hiv). Although in vitro studies provide some data on the interaction between antipsychotics and retroviral agents, there is a lack of in vitro data on this subject. We describe the case of a 35-year-old patient who suffered from schizophrenia, polydrug abuse and an hiv infection and who also had treatment-resistant psychosis. An interaction between the antiretroviral drug ritonavir and the antipsychotic olanzapine turned out to be the cause of the treatment resistance. As far as we know, this is the first report of such a clinically relevant interaction. We present a review of the current literature on this type of interaction.