Of 170 Western Australian patients who had their first A1DS-defining illness between 1 January 1983 and 31 December 1991. 61 (36%) were of unknown HIV antibody status (AIDS presenters), while 109 (64%) were of known HIV antibody status (HIV presenters). Pneumocystis carinii pneumonia (PCP) was less common as the AIDS-defining illness in HIV presenters (41% versus 62%. p 0.005). In this study of 70 patients with PCP as the index AIDS diagnosis. 36 were HIV presenters and 34 were AIDS presenters. Ten HIV presenters were taking prophylaxis at the time PCP manifested. The duration of symptoms of cough or dyspnea before the diagnosis of PCP was shorter, and the arterial Po2. measurement on admission was higher in those on prophylaxis, and a lower proportion of patients on prophylaxis required hospital admission (p - 0.05 for all comparisons). Furthermore. the CD4 counts at diagnosis of PCP were lower in patients taking PCP prophylaxis (mean 26 - 106L) than in patients who were not (mean 94 - 106L. p - 0.007). Of seven patients who died of PCP. none were receiving treatment for HIV disease before AIDS presentation. These findings suggest that PCP is prevented or deferred in patients receiving care for HIV disease and is less severe as a result of early diagnosis and treatment.