The x-ray findings of 727 patients with chronic interstitial nephritis are evaluated; these patients have been controlled over a period of several years: 594 suffer from chronic bacterial interstitial nephritis (pyelonephritis) and 133 patients from chronic abacterial interstitial nephritis of different etiology. The causes for the abacterial type of nephritis are phenacetin and primary gout. The radiological signs of the two forms of chronic interstitial nephritis in different degrees of involvement are pointed out. Whereas with the chronic abacterial interstitial nephritis symmetrical affection is typical, the chronic bacterial interstitial nephritis shows asymmetrical findings, especially in ascending pyelonephritis. Differentiation between the chronic bacterial nephritis and the chronic abacterial nephritis can be achieved in most cases by radiological signs, (morphological findings). The microscopic evaluation does not always allow a differentiation; because there are mixed forms and secondary bacterial infections are associated with primary chronic abacterial interstitial nephritis in the late stages. The multiple causes for chronic abacterial interstitial nephritis is radiologically reflected mostly by uniform signs during the different degrees of involvement.