Interleukin 1 secretion from human alveolar macrophages was studied in patients with interstitial pulmonary fibrosis, sarcoidosis, and the acquired immunodeficiency syndrome with pneumonitis and compared to secretion from alveolar macrophages of normal volunteers. Macrophages lavaged from the lungs were stimulated with 10 micrograms/ml of lipopolysaccharide and cultured for 24 hr. In some cases macrophages were also stimulated with 1 microgram/ml lipopolysaccharide. After dialysis of the culture supernatants, interleukin 1 secretion was quantified by the thymocyte proliferation assay and probit analysis and expressed in terms of secretion from 1 million macrophages. Results showed that, on average, macrophages derived from patients secreted more interleukin 1 after stimulation with lipopolysaccharide compared to normal subjects. Mean secretion was significantly greater from macrophages of patients with acquired immunodeficiency syndrome and interstitial pulmonary fibrosis when stimulated with 10 micrograms/ml lipopolysaccharide. Of the 24 individuals studied, spontaneous interleukin 1 secretion was detected from unstimulated macrophages in only 1 patient and 1 normal volunteer. We conclude that alveolar macrophages lavaged from the lungs of patients with inflammatory lung disease have an increased capacity to secrete interleukin 1 on in vitro stimulation with lipopolysaccharide. Possible mechanisms for this increase are discussed.