The objective of the study was to measure the utilization and diagnostic value of tests used in the Emergency Department (ED) on patients with undifferentiated non-traumatic abdominal or flank pain. Specific goals were to measure how often these tests led to changes in diagnosis or disposition, which tests were most commonly used, and which tests providers considered most helpful. We conducted a pilot single-center, prospective descriptive study, enrolling all eligible adult patients who presented to our ED with non-traumatic abdominal or flank pain during defined hours of our intake period. Based on serial provider interviews pre- and post-testing, we measured the frequency of change of most likely diagnosis and disposition, which tests were performed, and the provider-perceived value of tests. We enrolled 124 subjects with a mean age of 44 years; 27% were admitted. Testing led to a change in most likely diagnosis in 37% of subjects, and in disposition in 41%. Frequency of diagnostic test use varied from a high of 93% for CBC to 6% for a blood or urine culture. Overall, 65% of patients had at least one imaging study performed, and 39% had an abdominal/pelvic computed tomography (CT) scan. Over all subjects, providers identified the most useful tests as the CT scan (31%) and urinalysis (17%). In conclusion, among ED patients who presented with non-traumatic abdominal or flank pain to one academic center, the pre-test most likely diagnosis and disposition were changed based on the ED evaluation in over one-third of subjects. Almost all received blood tests and two-thirds received one or more imaging studies. Based on providers' subjective opinions, the most valuable tests were the abdomino/pelvic CT scan and the urinalysis.