BACKGROUND We emphasize the utility of focusing on patient medical history and concerns rather than anchoring on data sent from referral institutions, which is often qualitative and devoid of useful patient-driven information. CASE REPORT A 21-year-old man was referred to our hospital with persistent back pain, hypoalbuminemia, and C-reactive protein (CRP) elevation after prolonged hospitalization for a UTI at another hospital. A review of systems (ROS) revealed chronic diarrhea and colonoscopy revealed Crohn's disease. Colonoscopy was followed by worsening back pain. Intestinal perforation was ruled out by X-ray, and analgesics were prescribed for long-standing scoliosis. The patient returned several days later with a recurrent UTI; a vesicointestinal fistula was identified, a known complication of Crohn's disease. This case involved diagnostic errors due to the doctors' faulty cognitive process. Also, in retrospect, we needed to be aware that the CT at the time of referral showed free air in the bladder and not uncomplicated pyelonephritis. The diagnostic errors were related to both satisfaction bias (finding one disease that prevents the accurate and timely diagnosis of another) and lack of awareness of epidemiology. To prevent errors like these, it is important to first conduct a careful interview and physical examination, as if the patient were a first-time patient, in order to eliminate the influence of bias. Next, epidemiological possibilities should be considered and differentiation made between physical and epidemiological issues. CONCLUSIONS It is important to treat referral patients as if they were first-time patients and to give due consideration to diagnostic biases and epidemiology.