Proventriculitis was studied by experimentally reproducing the disease in broiler chickens. One-day-old infectious bursal disease virus (IBDV) and infectious bronchitis virus (IBV) antibody positive commercial broilers and 1-day-old antibody negative specific-pathogen-free (SPF) broilers were orally gavaged with proventricular homogenates produced from the proventriculi of broilers with proventriculitis. At 7 and 14 days, both commercial and SPF broilers had enlargement of the proventriculus with necrosis of the glandular epithelium and lymphocytic infiltrates in the proventricular glands. SPF broilers exposed to the proventricular homogenates developed infectious bursal disease, and IBDV was detected by reverse transcriptase–polymerase chain reaction (RT-PCR). They also were positive by RT-PCR to IBV and developed nephritis. Commercial broilers developed mild nephritis but not bursal disease and were negative for IBDV and positive for IBV by RT-PCR. Both homogenate-inoculated commercial and SPF chickens were negative for reovirus and Newcastle disease virus by RT-PCR and variably positive for adenovirus by PCR. Bacteria were not identified in histologic sections, nor were they isolated from affected proventriculi. Indirect fluorescent antibody assay using convalescent sera detected intracytoplasmic staining in the proventricular glandular epithelial cells. Examination of thin sections of proventriculi using electron microscopy identified virus-like particles approximately 120 nm in diameter within the cytoplasm of these cells at 7 days after inoculation. Passage of proventricular homogenate filtrates in chicken embryos for virus isolation caused stunting, and allantoic fluid from these eggs was positive for IBV by RT-PCR.