OBJECTIVE To determine the rate of virological rebound and factors associated with rebound among patients on highly active antiretroviral therapy (HAART) with previously undetectable levels of viraemia. DESIGN An observational cohort study of 2444 patients from the EuroSIDA study. METHODS Patients were followed from their first viral load under 400 copies/ml to the first of two consecutive viral loads above 400 copies/ml. Incidence rates were calculated using person-years of follow-up (PYFU), Cox proportional hazards models were used to determine factors related to rebound. RESULTS Of 2444 patients, 1031 experienced virological rebound (42.2%). The incidence of rebound decreased over time; from 33.5 in the first 6 months after initial suppression to 8.6 per 100 PYFU at 2 years after initial suppression (P < 0.0001). The rate of rebound was lower for treatment-naive compared with treatment-experienced patients. In multivariate models, patients who changed treatment were more likely to rebound, as were patients with higher viral loads on starting HAART. Treatment-naive patients were less likely to rebound. Among pretreated patients, those who were started on new nucleosides were less likely to rebound. CONCLUSION The rate of virological rebound decreased over time, suggesting that the greatest risk of treatment failure is in the months after initial suppression. Treatment-naive patients were at a lower risk of rebound, but among drug-experienced patients, those who added new nucleosides had a lower risk of rebound, as were patients with a good immunological response.