Twenty-seven cases of nephroblastoma were observed in 54 culled light meat broilers during a 1-year period. The tumours were generally unilateral and varied in size. Metastases were not observed. Histopathological studies showed epithelial and mesenchymal structures at different stages of development. Ultramicroscopy and immunohistochemistry findings revealed viral particles in intercellular spaces and in the lumen of the tubules not morphologically unlike avian leukosis/sarcoma viruses.