In chickens and presumably in other birds, some mast cell tumours may be metastatic. As increasing numbers of avian mast cell tumours are identified, more metastatic tumours may be identified and, since many birds lack lymph nodes, the lungs may prove to be a common site for metastatic lesions. The low mitotic index, lack of marked nuclear anaplasia and limited anisocytosis seen in this case may suggest that in chickens, in contrast to dogs, these characteristics may not be good predictors of metastatic potential. The present research project was on mast cell tumors in adult hen.