The incidence of nosocomial infection caused by extended-spectrum beta-lactamase (ESBL)-producing bacteria is increasing worldwide. Infections caused by ESBL producers have been associated with severe adverse clinical outcomes that have led to increased mortality, prolonged hospitalization, and rising medical costs. To avoid such adverse events and ineffective treatment, an appropriate use of drugs for infectious diseases is needed. To suppress the emergence and spread of drug-resistant bacteria in hospitals, it is important to be vigilant about ESBL-producing Escherichia coli (E. coli). In this study, we examined and compared seven items in a blood test between patients with ESBL-producing E. coli and non-ESBL-producing E. coli among febrile patients. We examined the levels of serum albumin, hemoglobin, and C-reactive protein (CRP), and the numbers of leucocytes, neutrophils, lymphocytes, and platelets in blood on the day of admission, the screening day during hospitalization, and the day immediately before discharge from the hospital. There were no significant differences in clinical background characteristics between the two groups of patients. In patients with invasive infections caused by ESBL-producing E. coli, serum albumin levels and the number of lymphocytes were significantly lower than those in patients not infected with ESBL producers. These values recovered to their baseline levels on the day of hospital discharge. This retrospective study suggests that serum albumin levels and the number of lymphocytes may serve as risk factors for infection by ESBL-producing E. coli, thereby supporting the appropriate use of antimicrobials in hospitals.