Agaricus brasiliensis currently is one of the most studied fungi because of its nutritional and therapeutic properties as an anti-inflammatory agent and an adjuvant in cancer chemotherapy. The effects of orally administered aqueous A. brasiliensis extract (14.3- and 42.9-mg doses) on parenchymal lung damage induced by carcinogenic 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were observed in Wistar rats. NNK treatment induced pulmonary inflammation, but not lung cancer, in the rats. The lungs of animals treated with NNK showed a higher level of inflammation than those of the control group according to histopathologic examinations (P < 0.01) and kurtosis analysis (P < 0.001) of a global histogram generated from thoracic computed tomography scans. There was no significant difference in the alveolar and bronchial exudates between animals treated with a 14.3-mg dose of A. brasiliensis extract and the control without NNK. However, a significant difference was found between animals treated with NNK, received a 42.9-mg dose of A. brasiliensis (P < 0.05), and the controls not treated with NNK. We did not observe a significant difference between the kurtoses of the A. brasiliensis (14.3 mg) and control groups. However, a 42.9-mg dose of A. brasiliensis resulted in lower kurtosis values than those observed in the control group (P < 0.001). In conclusion, a low dose of A. brasiliensis was more effective in attenuating pulmonary inflammation. Similar to the histopathological results, the computed tomography scans also showed a protective effect of A. brasiliensis at the lower dose, which prevented gross pulmonary consolidation.