To determine the frequency and clinical significance of ultrasonographically detectable alterations in renal volume and anatomy that are associated with acute pyelonephritis, 25 women underwent renal ultrasonography during the acute phase of their illness, and 21 of these patients underwent the procedure after receiving antimicrobial therapy. One patient had a predisposing anatomic abnormality (4%; 95% confidence interval, 0-12%), and one patient each (8%; 95% confidence interval, 0-19%) experienced focal complications (an intrarenal mass and perinephric fluid collection). The kidneys of the 21 evaluable patients were acutely swollen (mean, 20%; P =.0001); one or both kidneys were enlarged by >15% in 17 (81%). Acute renal enlargement was associated with protracted pretherapy symptoms (P <.01), leukocytosis (P <.01), focal infectious complications (P <.01), and prolonged hospitalization (P <.05). Thus, ultrasonographically demonstrable renal swelling characteristically occurs in women with acute pyelonephritis but is usually apparent only in retrospect. The degree of swelling correlates with selected clinical parameters, and the frequency of underlying anatomic abnormalities and focal infectious complications is low.