Objective - To compare concentrations of 3-methyleneindolenine (3MEIN) in lung tissues obtained from feedlot cattle that died as a result of acute interstitial pneumonia (AIP) and cattle that died as a result of other causes and to compare blood concentrations of 3MEIN in healthy feedlot cattle and feedlot cattle with AIP. Study Population - Blood samples and lung tissues collected from 186 cattle housed in 14 feedlots in the western United States. Procedure - Samples of lung tissues were collected during routine postmortem examination and submitted for histologic, microbiologic, and toxicologic examination. Blood samples were collected from cattle with clinical manifestations of AIP and healthy penmates. Histologic diagnoses were categorized as AIP bronchopneumonia (BP), control samples, and other disorders. Concentrations of 3MEIN were determined in lung tissues and blood samples, using an ELISA. Results - Concentrations of 3MEIN in lung tissues were significantly greater in AIP and BP samples, compared with control samples. Absorbance per microgram of protein did not differ between BP and AIP samples. Blood concentrations of 3MEIN were significantly greater in cattle with AIP, compared with healthy cattle or cattle with BR Odds of an animal with AIP being a heifer was 3.1 times greater than the odds of that animal being a steer. Conclusions and Clinical Relevance - Increased pulmonary production of 3MEIN may be an important etiologic factor in feedlot-associated AIP.