Proventriculitis is a problem affecting the processing of broiler carcasses, particularly those processed at 4 to 5 wk of age. The proventriculus and the gastric isthmus connecting the proventriculus to the gizzard are enlarged and swollen and often rupture during processing, causing carcass contamination. This study suggested that a filterable agent found in homogenated proventriculi can cause lesions similar to those seen in field cases. Proventriculitis was produced independently of an effect on growth, and only unfiltered homogenate caused stunting. Field birds with severe proventriculitis were shown to have increased body weights compared with birds without proventriculitis or with milder lesions. Intestinal weakness was not associated with proventriculitis and field birds with the most severe proventriculitis had stronger intestines. Although infectious proventriculitis has generally been reported as one of the lesions associated with stunting syndrome, these data suggest that it may have an independent etiology.