Proteinuria has been thought of as the first sign of serious renal disease. However, in some patients proteinuria may last for years without other evidence of kidney damage, and in other patients it may only be an insignificant and transient laboratory finding. The nature and severity of renal involvement in a particular patient are often suggested by the clinical picture, and the pattern and amount of proteinuria. The physician should use this information in evaluating the severity of the patient's disease. An approach for such evaluations is offered by briefly reviewing the causes and prognosis of the various types of proteinuria and by suggesting steps to take in treating patients with increased protein excretion.