Pneumocystis carinii colonization was studied in 90 men using the polymerase chain reaction. These comprised ten heterosexual controls; ten HIV-seronegative homosexual controls; 20 HIV-seropositive homosexuals with blood CD4 count > 400 x 10(6) l-1; 20 HIV-seropositive homosexuals with CD4 < 400 x 10(6) l-1; ten HIV-seropositive homosexuals with CD4 < 60 x 10(6) l-1 receiving PCP chemoprophylaxis; and 20 HIV-seropositive homosexuals with respiratory symptoms but without PCP. Induced sputum was obtained from all but the last group, who had bronchoalveolar lavage, and all specimens were tested for P. carinii using the polymerase chain reaction. The first four groups received no pneumocystis chemoprophylaxis, and all but the last group were asymptomatic. P. carinii colonization did not occur in the two control groups. P. carinii colonization rates were significantly different in the CD4 > 400, CD4 < 400, and CD4 < 60 groups (10%, 20%, and 40% respectively) (P < 0.025). Two patients (one each from CD4 < 400 and CD4 < 60) developed PCP 4-6 weeks after sputum induction, both had previously had high levels of P. carinii on sputum induction. Two patients from the CD4 < 400 group had high levels of P. carinii but did not develop PCP. In the symptomatic group, two subjects had low levels of P. carinii, but did not develop PCP. We have demonstrated P. carinii colonization in HIV-seropositive homosexuals in association with a low peripheral CD4 count. The polymerase chain reaction may be a useful technique for determining the need and efficacy of anti-pneumocystis chemoprophylaxis.