The aim of the study was to assess the prevalence of oral lesions in AIDS patients and identify associated factors. A cross-sectional study collected data from interviews, clinical examination, and a review of medical records for adult patients treated at the HIV/AIDS clinic in the University Hospital of the Federal University in Rio Grande, Rio Grande do Sul State, Brazil, focusing on socio-demographic, immune status, and treatment factors. Poisson regression was used in a hierarchical analytical model. From April 2006 to January 2007, 300 patients were observed (51% males; mean age 40 years). Of the total, 39% presented oral lesions, with candidiasis as the most frequent (59.1%), followed by hairy leukoplakia (19.5%). Women showed a lower risk of oral lesions, and there was an inverse association with CD4 count. Increased risk was associated with lower schooling, low income, smoking, alcohol addiction, time since HIV seroconversion, and higher viral load. The data confirm the increased prevalence of opportunistic oral lesions and show their relationship to socioeconomic conditions and modifiable habits and customs.