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Despite penicillin (pcV) treatment, tonsillopharyngitis caused by group A streptococci (GAS) is associated with bacterial failure rates as high as 25%. The reason for this rate of failure is not fully understood. One explanation might be that certain DNA profiles of GAS strains are responsible for treatment failures. Using arbitrarily primed polymerase chain reaction (AP-PCR), we compared the DNA profiles of GAS strains from 4 patients with several treatment failures following pcV treatment of tonsillopharyngitis with the profiles of strains of the same T type from patients who were clinically and bacteriologically cured after a single course of pcV. The isolates were obtained during the same time period and from the same geographic area. Thirty-seven strains of T types 4, 12, and R28 were investigated. Eleven different DNA profiles could be detected with the AP-PCR technique. Five DNA profiles were identified as T type 12, 3 as T type 4, and 3 as T type R28. The DNA profiles of the strains from the 4 patients with several treatment failures differed, but all isolates from each one of these patients exhibited the same or a very similar profile. The DNA profiles of the failure strains were also represented in nonfailure strains. Treatment failure in these 4 patients therefore seems to be due to insufficient eradication of GAS, rather than to reinfection with a new strain. The finding that the same DNA profile can be present in both failure and nonfailure strains suggests that the treatment failure may be to some extent host-related and not only due to bacterial factors.