When an infant develops acute respiratory failure of sufficient severity to necessitate supportive mechanical ventilation a cause should always be sought. A chest radiograph showing predominantly interstitial lung disease and an infant's failure to respond to standard antibiotic treatment are indications for non-bronchoscopic bronchoalveolar lavage. If P carinii pneumonia is diagnosed a congenital immunodeficiency should be sought and the parents counselled about HIV infection. Earlier investigation may be indicated by features of immunodeficiency when taking a history, performing a general examination, or analysing the results of basic haematological testing.