Objectives. Rituximab is an anti-CD20 monoclonal antibody that promotes better treatment outcomes in patients with non-Hodgkin's lymphoma (NHL). Case series of progressive multifocal leukoencephalopathy (PML) in patients receiving rituximab within polychemotherapy regimens have led to the introduction of a black box warning, but no risk estimation has ever been provided. Methods. We performed a retrospective, monocentric cohort study on 976 NHL patients diagnosed in 1994-2008, including 517 patients who received at least one dose of rituximab. Results. Inclusion of rituximab into standard chemotherapy regimens for NHL caused a significantly higher incidence of PML cases (rate difference, 2.2 every 1,000 patient-years; 95% confidence interval, 0.1-4.3). Interpretation. Based on this finding, clinical surveillance of PML-related symptoms is recommended in NHL patients exposed to rituximab.