All those clinical, laboratory, radiological and radioisotopic elements of importance in the diagnosis of chronic pyelonephritis in internal medicine have been examined. Anamnestic indications of chronic pyelonephritis include previous infections episodes of the urinary ways, pain in the lumbar and suprapubic region and micturition disturbances. Less significant are objective findings such as pain at percussion of the lumber region, hypertension and possible signs of renal insufficiency. Important diagnostically among laboratory examinations are leucocyturia higher than 1,000.000 in 24 hours, a findings of leucocyte cylinders, bacteriuria higher than 100.000 per cc of urine and the observation of immunoantibodies covering bacteria isolated from the urine. X-ray and radioisotopic examinations are recognised as having considerable diagnostic usefulness. None of the clinical, laboratory, X-ray or radioisotopic findings is, however, strictly specific. Confirmation of a diagnostic suspicion is only possible when scrupulous accumulation of the most typical subjective and objective findings regarding the disease is accompanied by the positivity of diagnostic tests. The results of these tests must in all cases be assessed critically in the context of the clinical picture for, taken singly, they have no decisive value.