Oral antimicrobial treatment of acute streptococcal pharyngitis commonly is given for 10 days. An investigation was conducted of journal publications and textbooks from the dawn of the antimicrobial era to the present in order to discover the basis for this settled practice. Current treatment duration for acute streptococcal pharyngitis was established half a century ago under conditions significantly different from those currently encountered by the average clinician. The 10-day treatment standard evolved without scientific justification. There have been no therapeutic trials that have validated the necessity for 10 days of oral antimicrobial treatment. Yet, despite the lack of a scientific foundation, the rarity of acute rheumatic fever in developed countries, the high failure rate for streptococcal eradication and evidence that short-course therapy with a nonpenicillin antimicrobial yields equivalent eradication rates, the 10-day rule persists because of long clinical usage and the inherent power of the number 10. Efforts to change this clinical benchmark will likely meet with resistance from practicing physicians.