The diagnosis of Pneumocystis carinii pneumonia (PCP) rests on the isolation of this micro-organism in patients whose latest blood count, less than 2 months old, shows less than 250 CD4 lymphocytes per cubic mm and who present with signs of impaired lung function. Bronchoalveolar lavage (BAL) is the reference diagnostic method, but induced expectoration may be the initial examination, in which case BAL is performed only when the latter fails or gives negative results. Prognostic factors are those of any interstitial pneumonia plus those specific to PCP and those associated with HIV infection. It is only when no initial severity factor is present that cure can be contemplated, provided the effectiveness of treatment is evaluated daily. Cotrimoxazole is the reference drug for comparisons with all new treatments; the indications of corticosteroid therapy and the necessity of intensive care techniques are now better determined. The frequency of PCP and its mortality rate should be reduced, and one may look forward to a time when this disease will be rare and atypical, thereby raising other diagnostic and therapeutic problems.