The gross and histopathologic changes produced by H. gallinarum in white leghorn chickens are described and compared with other common respiratory diseases. The histopathologic changes produced by H. gallinarum are acute inflammation of the turbinates and sinus epithelium, disruption of the trachea without cellular infiltration, and acute air sacculitis characterized by swelling and heterophilic response. These histologic changes appear to be different from those described for Newcastle disease, infectious bronchitis, infectious laryngotracheitis, and colibacillosis.