The authors studied two large-scale broiler chicken farms (A and B) where, contrary to the appropriate housing and feeding conditions, the feed consumption decreased, anorexia, retarded growth, anaemia and feather abnormality were experienced. On farm A, the rearing period extended by 4 days, the mortality rate increased by 4% and the mean slaughter weight decreased by 0.5 kg. On farm B, the rearing period did not, but the mortality rate increased by about 7%. The elevated mortality level appeared between 10-15 days and 10-21 days on farm A and B, respectively. Post mortem examination revealed that the lumen of the proventriculus, the gizzard and almost the whole length of the intestine was filled with brownish-red creamy content. In the gizzard, the overlaying keratinoid layer on mucosa was partly detached, brownish-red discoloured and bloody infiltrated. After removing the keratinoid layer, erosions of the mucosa were observed in a diameter of about 0.5-2.0 cm. Histopathological findings included the degeneration or desquamation of the keratinoid layer, necrotic mucosa, degeneration and disappearance of epithel cells, and oedematous lamina propria infiltrated by lymphocytes, macrophages and heterophils. In the affected epithel cells basophilic intranuclear inclusion bodies were observed. Cytopathogenic effect typical of adenoviruses was experienced on the inoculated tissue cultures. Presence of adenoviruses was confirmed by PCR in the supernatant of the passaged tissue culture and in the mucosa of the affected gizzards. The isolated adenoviruses proved to be FAdV-1 by phylogenetic analysis of the sequences of the PCR product. The judgement of the economic impact of the gizzard erosion is a complex problem. In order to get a correct diagnose, it is always necessary to consider all details, including the anamnesis of a flock, the results of clinical and post mortem examination, histopathology, virus isolation and identification.