BACKGROUND: The advent of antiretroviral therapy increased the life expectancy of human immunodeficiency virus (HIV)-positive patients and, consequently, the morbidity and mortality due to neoplasms. Plasmablastic lymphoma is one such neoplasm that generally presents with involvement of the oral cavity; cases of extra-oral involvement are rare.

CASE REPORT: We report a case of plasmablastic lymphoma in a 46-year-old woman for whom the initial clinical manifestation was a painless perineal tumor accompanied by fecal incontinence.

CONCLUSIONS: The possibility of this neoplasm should be considered in patients with HIV/acquired immune deficiency syndrome (HIV/AIDS) because its early diagnosis is essential so that the start of the treatment is not delayed.

Keywords: Diagnosis, Differential, Biopsy, Anus Neoplasms - diagnosis, HIV, HIV Infections - complications, Lymphoma, AIDS-Related - diagnosis, Magnetic Resonance Imaging, Neoplasms, Plasma Cell - diagnosis