Prompted by the observation of marked alkaline phosphatase (AP) elevation in a patient with severe, bilateral pyelonephritis, a retrospective survey was undertaken of the records of patients hospitalized during 1970–1972 under the headings of pyelonephritis, papillary necrosis, hydro- and pyonephrosis. Patients with liver and bone disease or chronic renal failure were excluded. Out of 95 consecutive patients studied, 14 (14.7%) had substantial elevations of serum AP. The common feature of most cases was pyelonephritis with severe systemic manifestations; four patients had gram-negative bacteremia; two patients died of sepsis and uremia and the autopsy findings included bilateral, widespread, nonobstructive pyelonephritis with papillary necrosis in one case and multiple renal abscesses in the other. Eight patients had associated uni- or bilateral hydronephrosis. It is postulated that inflammatory tubular damage and extra- or intrarenal obstruction can result in release of AP into the circulation. Elevation of serum AP in pyelonephritis may be used as an indicator of extensive paren-chymal destruction and/or urinary tract obstructio