Pneumocystis pneumonia (PCP) is one of the infectious lung diseases diagnosed in HIV-infected patients. The pathogen responsible for the development of this opportunistic infection is an atypical fungus called Pneumocystis jiroveci. PCP remains the most common disorder diagnosed at the onset of acquired immunodeficiency syndrome (AIDS), especially in individuals not aware of their HIV infection. The most important risk factor of PCP development in HIV-infected person is the decrease of T CD4+ cell number below 200/mcL. Clinical symptoms consist of: chronic cough, dyspnoea and weakness. Arterial blood gas analysis often reveals the presence of hypoxaemia, and high-resolution computed tomography imaging shows diffuse ground glass opacities. Treatment is based on intravenous administration of trimetoprim-sulfamethoxasole. In patients with moderate and severe symptoms of PCP it is recommended that corticosteroids are used. Co-infection with cytomegalovirus (CMV) is a poor prognostic sign in PCP. CMV-related organ disease should be suspected in patients with T CD4+ cell number lower than 50/mcL. It is arguable whether CMV infection in symptomatic PCP patients should be treated. There are suggestions that corticosteroids used in PCP patients with CMV co-infection could promote the development of CMV pneumonia. In the present paper we present two patients with PCP, unaware of their HIV infection. In both cases a CMV co-infection was found.