Pneumonia is one of the most significant diseases affecting livestock, including sheep. This disease manifests in various forms, causing inflammation and damage to the lungs. Several factors contribute to its occurrence, making it a complex condition to manage. This study was conducted to investigate the prevalence of pneumonia in sheep and to identify the bacterial agents responsible. The research took place in the spring of 1401 at a slaughterhouse in Tehran, where 98 lung samples from slaughtered lambs exhibiting macroscopic symptoms of pneumonia were randomly selected from densely raised lambs. Microscopic tests and histopathological analysis revealed seven distinct patterns of pneumonia. The frequency of these patterns was as follows: 18 cases of acute catarrhal bronchopneumonia, 14 cases of purulent catarrhal bronchopneumonia, 9 cases of purulent necrotic bronchopneumonia, 26 cases of fibrinous pneumonia, 6 cases of fibrinous necrotic bronchopneumonia, 18 cases of interstitial pneumonia, and 6 cases of worm pneumonia. Pneumonia remains a leading cause of epidemics and mortality in this region, significantly impacting the health and productivity of the livestock. The study identified Mannheimia hemolytica (56.14%), Escherichia coli (24.56%), and Pasteurella multocida (10.52%) as the primary bacterial agents involved in lamb pneumonia in this area. These pathogens contribute to the severity and spread of the disease, complicating treatment efforts. The histopathological characteristics of pneumonia vary depending on factors such as the specific etiological agents, host immunity, environmental conditions, treatment measures, and the disease''s prognosis. This research highlights the ongoing challenges in managing pneumonia in livestock and underscores the importance of continued surveillance and targeted interventions to mitigate its impact. Understanding these factors is crucial for developing effective prevention and treatment strategies, thereby improving the overall health and productivity of the sheep population.