Systemic karyomegaly associated with interstitial nephritis was first described in 1978 by Mihatsch. Seven cases have been reported to date. We give an account of an autopsy case of systemic karyomegaly in a 30-year-old Italian man. Bizarre enlargement of nuclei was found in renal tubular epithelial cells, Schwann cells and in smooth muscle cells of vessels and bowel and, less obviously, in endothelial and adventitial cells of vessels, in alveolar epithelial cells and in astrocytes of the brain. These findings were associated with chronic interstitial nephritis, nonspecific hepatopathy, adenocarcinoma of the rectum and multiple sclerosis. The clinical course was marked by chronic renal failure, chronic haemodialysis and renal transplantation. The patient died 8 years after diagnosis in septic-toxic shock. The aetiology and pathogenesis of the disease are discussed.