['To analyse whether blips are associated with a higher risk of virological or immunological failure than persistent undetectable viraemia (UND) among HIV-infected patients receiving HAART.', 'Retrospective cohort study.', 'Patients with blips or UND were selected from a prospective cohort of 330 patients under HAART for over 48 weeks. Blips were defined as detectable viraemia up to a maximum of 1000 copies/ml preceded by two consecutive visits and followed by one visit showing undetectable viraemia. Patients were included according to the following criteria: i) Blip group: patients that showed transient relapses of viraemia; ii) UND group: patients who had achieved UND on HAART before 24 weeks of therapy and that sustained viral suppression for four consecutive visits.', 'Virological and immunological failure.', 'Thirty seven (11%) and 65 (20%) patients showed blips and persistent UND, respectively. Virological failure was observed in three (8.1%) patients in the blip group and 11 (16.9%) patients in the UND group (P=0.25). The time to virological failure was shorter in the UND group (P=0.12). The rates of virological failure and the time to virological failure were similar between both groups after excluding patients with compliance <95%. The time to immunological failure was also similar in both groups (P=0.5). In a Cox model, only the use of saquinavir hard gel-based regimens was independently associated with the time to virological and immunological failure.'] Are transient rebounds of low-level viraemia among HIV-infected patients under HAART associated with virological or immunological failure?