On the assumption that increased urinary lysozyme concentration (`lysozymuria') indicates tubular proteinuria and therefore impaired tubular function, urinary lysozyme has been estimated in acute disorders where transient disturbances of renal function might be expected, in cases diagnosed clinically as extrarenal uraemia, and in a few examples of acute renal disease. Reversible lysozymuria occurred with hypokalaemia, postoperative `collapse', electrolyte depletion, severe extrarenal infection, acute pyelonephritis, the nephrotic syndrome, after a few apparently uncomplicated surgical operations, and very transiently after ventricular fibrillation abolished by DC shock. There was no lysozymuria with severe uraemic heart failure, aspirin and paracetamol poisoning, or severe jaundice, nor in two cases of acute glomerulonephritis. Although lysozymuria may occasionally be useful in the clinical diagnosis of acutely disordered renal function, the results suggest that its value is limited; on the other hand, they have provided information on renal pathophysiology in acute disease.