After 19 weeks on a mash diet nine of the 10 birds exhibited from two to 19 oral lesions. The majority of these lesions involved the total erosion of the buccal epidermis giving a total area of exposed dermis of 0.6 to 94.5 mm(2). This was accompanied by a significant reduction in the numbers of heterophils, monocytes and eosinophils. Birds with lesions also had significantly reduced concentrations of haemoglobin and may have been suffering from a mild form of microcytic normochromic anaemia without the complication of an extensive bone marrow involvement. These findings have implications for poultry welfare.