In Denmark a nephropathy in pigs characterized by tubular atrophy and interstitial fibrosis has been identified frequently during the last 5 decades in the course of meat inspection in slaughterhouses. The disease was first described by Larsen, who recognized the connexion between feeding mouldy rye to pigs and the development of the nephropathy. In this study kidneys were examined from 19 pigs coming from a farm with an outbreak of nephropathy. The barley fed to the pigs was contaminated with the mycotoxin ochratoxin A. Histological examination revealed different degrees of change ranging from slight regressive changes in the tubular epithelium and periglomerular and interstitial fibrosis to tubular atrophy, thickened basement membranes, glomerular sclerosis, and marked fibrosis. These differences were considered to be due to differences in the length of time of exposure to the mouldy barley and differences in the amount of mycotoxin consumed by the individual pig. However, it will be necessary to carry out experiments using crystalline ochratoxin A in order to prove such a relationship. Mycotoxins have also been suggested as etiological factors in Balkan nephropathy in man, which in the initial stages is characterized by tubular lesions similar to those seen in mycotoxic nephropathy in pigs.