Abstract: The purpose of the present investigation was to study the histological findings in a tooth diagnosed with acute pulpitis in a patient suffering from AIDS. Histological investigation of the pulp revealed that decay progress caused inflammatory response of the underlying pulp with different features compared to the inflammatory response found in the teeth of non-diseased individuals. The study revealed different kinds of chronic inflammatory cells and absence of polymorphonuclear leucocytes. Furthermore, widely dilated pulpal blood vessels and formation of zones, where the pulpal tissue was absent, were observed, as well as, traces of hard tissue of the osteotypic form. The inflammation was found to follow the Van Hassel model (expansion of the inflammatory reaction from the primary inflamed area towards the apices).