Objective - To evaluate viral and bacterial respiratory pathogens and Mycoplasma spp isolated from lung tissues of cattle with acute interstitial pneumonia (AIP) and cattle that had died as a result of other causes. Sample Population - 186 samples of lung tissues collected from cattle housed in 14 feedlots in the western United States. Procedure - Lung tissues were collected during routine postmortem examination and submitted for histologic, microbiologic, and toxicologic examinations Histologic diagnoses were categorized for AIP, bronchopneumonia (BP), control samples (no evidence of disease), and other disorders. Results - Cattle affected with AIP had been in feedlots for a mean of 127.2 days before death, which was longer than cattle with BP and control cattle. Detection of a viral respiratory pathogen (eg, bovine respiratory syncytial virus [BRSV], bovine viral diarrhea virus, bovine herpesvirus 1, or parainfluenza virus 3) was not associated with histologic category of lung tissues. Bovine respiratory syncytial virus was detected in 8.3% of AIP samples and 24.0% of control samples. Histologic category was associated with isolation of an aerobic bacterial agent and Mycoplasma spp. Cattle with BP were at greatest risk for isolation of an aerobic bacterial agent and Mycoplasma spp. Conclusions and Clinical Relevance - Analysis of these results suggests that AIP in feedlot cattle is not a consequence of infection with BRSV The increased risk of isolation of an aerobic bacterial agent from cattle with AIP compared with control cattle, may indicate a causal role or an opportunistic infection that follows development of AIP.