Several factors are known to upregulate in vitro HIV expression by infected T cells, such as certain cytokines and cell-cell interactions. The effect of such factors, involved in immune responses, has never been evaluated in relation to HIV production by infected cells in vivo. To do so, we assessed HIV production by enriched alveolar and blood lymphocytes using a quantitative p24 antigen coculture. This was performed in 32 HIV-seropositive subjects in relation to their pulmonary infectious status, that is, with no current infection (Group 1, n = 17) or with Pneumocystis carinii pneumonia (PCP) (Group 2, n = 15). We showed that HIV core p24 antigen production by enriched alveolar lymphocytes is strongly related to the presence or absence of PC. Although the prevalence of p24-positive alveolar lymphocytes cocultures was 100% in subjects with PCP (15 of 15) compared with 65% in those without lung infection (p = 0.02), all blood lymphocytes tested were positive. In addition, the viral production of alveolar lymphocytes was 40-fold and 8-fold increased at Days 4 and 6 of the culture period in Group 2 compared with Group 1, respectively (p < 0.005). Finally, p24 antigen production by alveolar lymphocytes was significantly higher than that by the corresponding blood lymphocytes in subjects with PCP (p = 0.03) and lower in subjects without lung infection (p = 0.025). Altogether, these data strongly suggest that the HIV burden is markedly enhanced in the lung during PCP. This increased viral production is, at least initially, compartmentalized to the affected organ.