Population-based rates of hospitalization for acute pyelonephritis were estimated over 3 years with use of provincial administrative data on health care. Retrospective review of records of ICD9-CM discharge codes of pyelonephritis and urinary infection was performed in two acute care institutions to validate the discharge diagnosis; 80% of patients with a discharge diagnosis of pyelonephritis and 7% to 20% of patients with a discharge diagnosis of urinary infection met clinical criteria for admission for acute pyelonephritis. Mean rates of hospitalization +/- SD for acute pyelonephritis were 10.86 +/- 0.51 per 10,000 population among women and 3.32 +/- 0.27 per 10,000 population among men. From 18% to 31% of hospitalized women aged 20-39 years pregnant; 36% of 797 hospitalized women and 21% of 402 hospitalized men 40 years of age or older were diabetic. The hospitalization rates among Native American women with treaty status were five to 20 times greater than those among other women, which was partially attributable to a significantly greater frequency of pregnancy and diabetes in the former women. Hospitalization for acute pyelonephritis is common, and pregnancy and diabetes contribute substantially to hospitalization rates. The increased hospitalization rate among Native American women with treaty status is not fully explained by pregnancy or diabetes.