A condition showing nephrosis-nephritis caused by infectious bronchitis virus (IBV) and atrophy of the bursa of Fabricius attributable to infectious bursal disease virus (IBDV) occurred with a mortality rate of 6.4%, 7.4% and 8.5% in young broiler chickens in three flocks on a poultry farm. Eighteen birds from the outbreaks were studied histologically and ultra-structurally. Nephrosis-nephritis appeared initially as degenerative changes of the convoluted tubules, followed by inflammatory reactions in the affected parenchyma and interstitium. Virus particles similar to IBV were demonstrated in the epithelial cells and lumens of the convoluted tubules. The bursa of Fabricius revealed lesions characteristic of infectious bursal disease and virus particles identical to IBDV were found in the reticulo-epithelial cells of the follicles. Two-thirds of the birds examined had the same histological lesions in the larynx and trachea as those seen in infectious bronchitis. The possibility existed of a pathogenetic interaction between IBV and IBDV infections.