Objective To study the histopathological changes in lungs spontaneous lesions of breeding %macaca mulatta.% Methods Lung samples from 2-20 years monkeys were investigated by routine histological technique.Results Various spontaneous lesions,totaling 17 kinds in different degrees,were found in 84.6% monkeys of the study,including pneumonia,inflammatory cell infiltration,degenerations,abscess,proliferation of fibrous tissue, and fibrosis.The degree and the freqency of lesions increased with age.Conclusion The spontaneous lesions of lung were often observed in %macaca mulatta% and its interference effect should be seriously taken.