BACKGROUND Inflammation-associated proteinuria in acute, nonrenal disease is a common but poorly understood phenomenon. We performed an observational study of the urinary excretion of orosomucoid (alpha(1)-acid glycoprotein), albumin, alpha(1)-microglobulin (protein HC), and IgG to obtain quantitative and temporal data on these 4 proteins. METHODS Urine samples were collected at daily intervals for up to 23 days from 6 patients with surgery-induced inflammation and at hourly intervals for a 24-h period from 7 sepsis patients. Urinary protein concentrations were assessed by immunoturbidimetry. RESULTS During surgery-induced inflammation, the increase and decrease in orosomucoid excretion mirrored changes in plasma C-reactive protein. Values for all 4 urinary proteins were increased in sepsis patients. The observed maximum increases in urinary protein excretion relative to the upper reference values were 280-fold for orosomucoid, 98-fold for alpha(1)-microglobulin, 33-fold for albumin, and 26-fold for IgG. CONCLUSIONS Orosomucoid, usually present in plasma and urine in much lower concentrations than albumin, is increased in urine to concentrations equal to or higher than albumin in proteinuria associated with acute inflammation. The pathophysiologic mechanisms responsible for this markedly increased excretion are unknown. Monitoring of urinary excretion of orosomucoid and other specific proteins, expressed as protein/creatinine ratios, may provide a window for clinically relevant real-time observation of changes in acute inflammatory processes. Orosomucoid in urine may be a more informative marker than albumin for inflammation.