The maintenance of undetectable levels of human immunodeficiency virus (HIV) viraemia seems to assure sustained benefit of antiretroviral therapy, since ongoing virus replication under drug pressure offers the opportunity for resistant variants to emerge and replicate. The early recognition of even low levels of virus replication should be helpful, allowing clinicians to consider switching therapy before accumulation of drug-resistant mutations has occurred. In this context, ultrasensitive assays could be very helpful. The new nucleic acid sequence-based assay (NASBA) (Nuclisens, Organon-Teknika), which has a limit of detection of 20 HIV RNA copies/ml, was tested in clinical specimens from 42 HIV-infected patients receiving different antiretroviral treatments and having undetectable levels of plasma viraemia (<500 copies/ml) using another less sensitive quantification method. Overall, 21 (50%) samples were positive and yielded quantitative values using Nuclisens. Since complete suppression of virus replication may be necessary to avoid the development of drug-resistant mutants, the introduction of ultrasensitive methods for monitoring response to treatment should be recommended when eradicative strategies are pursued.