BACKGROUND: Our goal was to determine the incidence rate and risk factors for loss to follow-up (LTFU) of HIV-infected patients in Northern France. METHODS: We estimated the incidence rate of LTFU in 1,007 HIV-infected patients under care from January 1997 to December 2006. We then investigated potential risk factors for LTFU at inclusion and during follow-up. RESULTS: The incidence of LTFU was estimated to be 3.5 per 100 person-years. Risk factors for LTFU at enrolment in a multivariate Cox model were age <30 years (hazard ratio [HR] 1.66 versus >40 years, 95% confidence interval [CI] 1.04-2.64), transmission by injection drug use (HR 5.26 versus men who have sex with men, 95% CI 2.90-9.52), no phone number provided (HR 5.4, 95% CI 3.6-8.2), no primary care physician (HR 2.10, 95% CI 1.25-3.52) and sub-Saharan African origin (HR 2.09, 95% CI 1.36-3.22). Patients with CD4(+) T-cell counts <200 cells/mm(3) (HR 0.49 versus >/=350 cells/mm(3), 95% CI 0.32-0.76) and 200-349 cells/mm(3) at baseline (HR 0.63 versus >/=350 cells/mm(3), 95% CI 0.41-0.98) had a decreased risk of LTFU. During follow-up, the risk of LTFU increased when the most recent CD4(+) T-cell count was <200 cells/mm(3) (HR 2.06, 95% CI 1.16-3.66), the patient was not on highly active antiretroviral therapy (HAART; HR 4.20, 95% CI 2.66-6.61) and the patient was on HAART but had a detectable viral load (HR 1.92, 95% CI 1.19-3.01). CONCLUSIONS: Our findings will help clinicians recognize patients who require additional support for retention in care, including younger patients, injection drug users, people of sub-Saharan African origin, patients who are healthier at enrolment and patients who do not adhere to HAART during follow-up.(R.A.)