We have attempted to elucidate the natural history of pulmonary aspergillus intracavitary colonization (PAIC) based on more than 350 cases of the disease observed in the last 11 years and on data collected from the literature. The data indicate that PAIC is a dynamic process consequent to the continual growth and death of fungal elements and also with their relationships to the anatomic features of the cavity (valvular mechanisms, vascular alteration). The clinical presentations reflect immunological changes in the host. Metabolites produced by the species of Aspergillus involved affect the clinical presentation of the syndrome.