Ochratoxin A is suspected of being one of the etiological agents responsible for Balkan endemic nephropathy and the associated urinary tract tumours. Contamination of cereals by this mycotoxin has been found to be more frequent in areas of endemic nephropathy than in areas where the disease is absent. As ochratoxin A binds to serum albumin, it should be detectable in biological fluids from exposed populations. A survey was thus conducted to determine the occurrence of ochratoxin A in blood from people living in the endemic area who were either affected or unaffected by the two diseases and in blood from people living in control regions where these diseases do not occur. Blood samples were collected in 1984, 1986, 1989 and 1990. Ochratoxin A was found more frequently and at higher levels in blood from patients with Balkan endemic nephropathy and/or urinary tract tumours than in blood from unaffected people from endemic and control areas. These findings suggest further that ochratoxin A is involved in the etiology of the two diseases.