Background: Gross hematuria is a screening test used to help determine the need for radiographic evaluation in patients with blunt trauma. This subjective assessment has not been compared with objective measures, nor has interrater reliability been described. Methods: We performed a prospective, randomized, controlled study to determine the ability of clinicians to assess gross hematuria. Clinicians were asked to assess samples containing various concentrations of blood diluted in urine for the presence of gross hematuria. Results: Gross hematuria was recognized by more than 95% of clinicians only when samples contained more than 3,500 red blood cells per high-power field. Clinicians' interpretations were independent of profession, specialty, and level of training (p > 0.08). Conclusion: Clinicians demonstrate poor sensitivity and interrater reliability identifying gross hematuria by inspection. Clinical studies correlating degrees of hematuria to the need for genitourinary diagnostic evaluation should report both subjective and objective measures to identify a threshold value predictive of renal injury.