A seasonal respiratory disorder occurring among Saudi Arabs with certain clinical manifestations resembling classic Löffler's syndrome is described. Clinical and epidemiologic evidence indicate that this entity is a form of larval ascariasis. The sharp seasonal incidence of the syndrome is believed due to special climatic factors that interrupt the transmission of ascariasis. The pathogenesis of the syndrome is likely to be due in part to hypersensitivity phenomena resulting from repeated migrations of larvae. Serologic tests with whole-worm antigens have not proved reliable in detecting evidence of recent infection with A. lumbricoides under the conditions reported in this study. The most helpful criteria by which seasonal pneumonitis can be distinguished from a number of other conditions that have in common pulmonary infiltrations and eosinophilia are: the clinical findings, the seasonal incidence, and the finding of larvae of A. lumbricoides either in the sputum or gastric washings, or in both.