Bacteremia develops in a subgroup of patients with acute pyelonephritis. This study examined isolates of Escherichia coli from the urine and the blood of 25 bacteremic and 67 nonbacteremic women with this acute disease. P-fimbriated strains were found in 100% of bacteremic patients without complicating factors but in only 71% of nonbacteremic patients without complications (P < .05). Non-P-fimbriated strains were only found to cause bacteremia in three patients with compromising host factors. Strains from the bacteremic group and those from the nonbacteremic group did not differ significantly in terms of hemolysin or aerobactin production or of serum resistance. The P-fimbriated strains from both groups of patients carried pap DNA sequences of the papGIA2 adhesin type; prsGJ96 homologous DNA sequences were rare. The results suggested that P fimbriae and compromising host conditions independently increase the risk for bacteremia during acute pyelonephritis.