The incidence of Pneumocystis carinii pneumonia (PCP) during 2 years in HIV-infected patients with less than or equal to 100 x 10(6)/l CD4+ cells, less than or equal to 200 x 10(6)/l CD4+ cells and less than 20% CD4+ cells of total T lymphocytes were compared. The relative PCP risk in 57 patients with less than or equal to 100 CD4+ cells was more than twice higher than in 120 patients with less than or equal to 200 CD4+ cells. The latter had almost twice higher relative PCP risk than 271 patients with less than or equal to 20% CD4+ cells. Only 3/56 patients who acquired PCP had greater than 200 CD4+ cells and 15/56 patients had greater than 100 CD4+ cells. Centers for Disease Control (CDC) recommends primary PCP prophylaxis in HIV-infected patients when the number of CD4+ cells is less than 200 x 10(6)/l or when the CD4+ is less than 20. On the basis of the presented data we suggest that primary prophylaxis is considered only when CD4+ cells fall below 200 x 10(6)/l.