The serum electrolyte panel (SEP) is commonly ordered in the workup of the emergency department (ED) patient. This study was done: 1) to evaluate the efficacy of the SEP in terms of the identification of clinically significant abnormals (yield) and the impact on therapeutic plan (impact); 2) to evaluate the reasons that the test was ordered; and 3) to compare the expected and realized contributions of the test to patient care. Pretest and posttest questionnaires were administered to physicians managing 800 ED patients greater than or equal to 55 years old for whom SEPs were ordered. The yield of significant abnormals was 16%. Fluid and electrolyte treatment plans were modified after the SEP results became known in 35% of cases. This modification was associated with a normal SEP 48% of the time. Both the yield and the impact of the SEP were related to the reason that the test was ordered. The most common reason given was "to look for an unexpected abnormality" (50%). Physicians' expectations for the contribution of the SEP to patient care decisions were greater than the contributions realized after the results were known. However, in 115 cases, the test contributed more than expected. Physicians predicted that 13% of the tests would contribute nothing to patient care. After the results were known, they felt that 38% had made no contribution. Physicians tend to overestimate the potential impact of the SEP but are occasionally surprised by a result that contributes more than expected. Thus, there is considerable pretest uncertainty about treatment decisions and normal results appear to have a substantial impact.