Bronchoalveolar lavage (BAL) cytology and immunoglobulin components of lavage fluid were studied during non-bacterial/non-fungal interstitial pneumonitis, bacterial/fungal pneumonia, and Pneumocystis carinii infection. Lavages done before bone marrow transplantation and in asymptomatic phases were used as controls. The total cell recovery was increased during lung processes of any aetiology. Non-bacterial/non-fungal pneumonitis caused a significant increase in the number of lymphocytes; the number of neutrophils increased particularly during bacterial pneumonia. In Pneumocystis carinii pneumonia the typical cell picture was an increased percentage of lymphocytes together with blasts. During acute graft-versus-host disease without respiratory symptoms the cytology in lavage fluids did not differ from the controls. Cytomegalovirus (CMV) isolation was frequently positive in lavage fluids regardless of the presence or absence of pulmonary symptoms, but most of the symptom-free CMV-positive patients did not have any marked changes in BAL cytology. The albumin content of BAL fluid increased during infectious and immunological processes in lungs.