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Objective—To determine the prevalence and temporal onset of lung lesions in lambs and the impact of lung lesions on growth of affected lambs. Animals—259 crossbred wether lambs from a single flock in the upper Midwestern United States. Procedure—An observational study was conducted. Lambs born in the spring and fall were slaughtered at finished weight or at a predetermined time point. Lungs of each lamb were examined and classified as normal, moderate lesions (consolidation > 5% but ≤ 50% of any lobe), or severe lesions (consolidation > 50% of any lobe). Data were examined to detect effects of prevalence or severity of lung lesions on growth and carcass traits. Results—57 of 89 (64%) spring-born lambs had lung lesions characterized by consolidation of lung tissue. A small number of lambs had pulmonary adhesions or active abscesses. In contrast, only 31 of 108 (29%) fall-born lambs had lung lesions. Severe lung lesions were associated with a significant reduction in average daily gain. Severe lung lesions were not detected until the middle of the finishing period and were associated with culture of Mannheimia haemolytica or Pasteurella multocida. Conclusions and Clinical Relevance—Analysis of results indicates that the prevalence of severe lung lesions can be quite high in lambs. Severe lung lesions can lead to greatly decreased growth performance of lambs.