Escherichia coli was the most frequent species isolated from blood cultures in the Hospital Covadonga of Oviedo (Spain) over a six-year period (474 episodes, 15.3% of the total septicemias and 2.7 episodes per 1,000 patients). Escherichia coli strains were susceptible in greater than 95% of episodes to cefoxitin, cefotaxime, gentamicin, tobramycin and amikacin. In a series of 72 episodes, microbiological features and host factors were studied. No endemic stains were found. Type 1 fimbria was detected in 73.6% of strains and P-fimbriae in 12.5%, without correlation between P-fimbria and urinary infection; 84.7% of the strains were resistant to decomplement human serum; 61.1% produced aerobactin and 20.8% were hemolytic. Factors such as age, hospital location, metastatic focus and surgical treatment were significantly correlated with morbidity and mortality. The global mortality rate was 18%, and in 8.3% of cases was directly associated with septicemia.