Farmer's lung disease (FLD) is a hypersensitivity pneumonitis secondary to the inhalation of moldy hay spores. Its prevalence is likely underestimated despite the fact it may result in significant acute and chronic respiratory disability. The immunologic mechanisms are best explained as Gell and Coombs Type III & IV reactions. FLD is usually recognized by history and appropriate laboratory confirmation. Therapy requires removal of the patient from the offending antigens, although corticosteroids may be useful for constitutional symptoms.