Sixteen bronchoalveolar lavages (BAL) were performed in 15 children with HIV1 seropositivity, 12 of them being infected by HIV1. BAL was performed during episodes of acute pneumonitis with respiratory distress (group I: three cases) or without severity (group II: five cases), or in the presence of asymptomatic radiological pulmonary abnormalities (group III: seven cases). A specific diagnosis of infection was obtained in five cases of acute pneumonitis and 12 micro-organisms were identified by BAL: three cytomegaloviruses, three respiratory syncytial viruses, two Pneumocytis carinii, one Haemophilus influenzae, one Herpes simplex virus type 1, one Escherichia coli and one group A streptococcus. In three cases two micro-organisms were simultaneously identified. Cytological examination showed a high proportion of polymorphonuclear leukocytes in cases of acute pneumonitis (group II) and alveolar lymphocytosis in clinically asymptomatic children with radiological pulmonary abnormalities (group III). BAL appears to be a reliable tool for the investigation of pulmonary infections in children with HIV1 seropositivity. In addition it has the advantage of revealing latent cytological abnormalities in these patients.