Influenza A virus load was assessed by real-time polymerase chain reaction (PCR) in nasopharyngeal swabs of infected patients treated with oseltamivir. The mean pretreatment virus load was significantly lower in the 24 patients (group A) who initiated treatment within 24 h of the onset of symptoms than it was in the 26 patients (group B) who initiated treatment between 24 and 48 h (1.6×105 vs. 8.4×105 copies/600 ng of total RNA, P=.04); after 48 h of treatment, twice as many patients in group B still had a positive PCR result, compared with patients in group A (42.3% vs. 20.7%). These virological results support the clinical benefit provided by early therapeutic intervention of influenza illness