Acute pharyngitis in adults is primarily a viral infection; only about 10% of cases are of bacterial etiology. Most cases of bacterial pharyngitis are caused by group A beta-hemolytic streptococci (GABHS). One laboratory method for the diagnosis of GABHS is rapid antigen diagnostic testing (RADT), which can be processed during an emergency department visit and which has become a popular alternative to throat swab cultures. We conducted a study to define the sensitivity and specificity of RADT, using throat culture results as the gold standard, in 100 emergency department patients who presented with symptoms consistent with streptococcal pharyngitis. We found that RADT had a sensitivity of 68.2% (15 of 22), a specificity of 89.7% (70 of 78), a positive predictive value of 65.2% (15 of 23), and a negative predictive value of 90.9% (70 of 77). We conclude that RADT is useful in the emergency department when the clinical suspicion is GABHS, but results should be confirmed with a throat culture in patients whose RADT results are negative.