OBJECTIVE: To determine whether presenting test performance characteristics influences clinical management. DESIGN: Two questionnaire-based, randomized controlled trials. SETTING: Mailed surveys with 2 clinical vignettes. PARTICIPANTS: Randomly selected US pediatricians (N=1502). INTERVENTION: Vignette-specific, randomly assigned test information: no additional information (control), test characteristics (TC), or TC defined. In the pertussis vignette, the TC group received the direct fluorescent antibody test's sensitivity and specificity, and the TC defined group received the same information with definitions. In the urinalysis vignette, the TC group received the false-positive rate of persistent microhematuria in predicting renal disease, and the TC defined group received a definition of this information. MAIN OUTCOME MEASURES: In the pertussis vignette, diagnostic test choice and management of erythromycin therapy and hospital discharge plans. In the urinalysis vignette, serum laboratory testing and nephrology referral plans. RESULTS: Six hundred fifty-three participants (49.5% of those eligible) returned completed surveys. In the pertussis vignette, significantly more of the TC (73%) and TC defined (71%) groups ordered the best-performing test than did controls (21%) (P<.001 for both comparisons). Receiving test characteristics did not significantly affect erythromycin therapy or hospital discharge plans (P>or=.40). In the urinalysis vignette, the TC defined group referred to nephrology (30%) and checked laboratory tests (88%) significantly more often than did controls (19%, P=.01; 78%, P=.01, respectively), but the TC and control groups' testing and referral plans did not differ significantly (22% vs 19%, P=.36; 75% vs 78%, P=.48, respectively). CONCLUSION: Providing test performance characteristics influenced certain clinical decisions, sometimes in unexpected ways.