Agenesis of the gallbladder is a relatively rare congenital anomaly with an incidence of only 0.01% to 0.04%. Yet it continues to occur. The patient described in this case study was symptomatic and received a clinical diagnosis of cholecystitis after having an ultrasound examination that was read as indicating a small contracted gallbladder with stones. Only after an open surgical procedure and cholangiography was it discovered tht the patient actually had a congenital absence of the gallbladder. Clinicians need to be aware of the potential for congenital absence of the gallbladder when interpreting tests and when findings are questionable, and to ensure no surprises, they should consider ordering other diagnostic tests before doing surgery.