In this case series, hospitalized patients who tested positive for 2009 pandemic influenza A (H1N1) received oseltamivir within 2 days of symptom onset and appeared to recover quickly with a median hospital stay of two days. Similarly, no ambulatory patients treated with oseltamivir required further medical care. In contrast, 3 patients initially discharged from the ED without oseltamivir returned to triage and required hospital admission. Two of these 3 later tested positive for 2009 pandemic influenza A (H1N1). One additional patient who required mechanical ventilation and subsequently died had presented 6 days after symptom onset. Another 5 hospital decedent whose care was transferred to the infectious disease service and therefore not part of our triaged case-series presented a median of 15 days after symptom onset. These cases suggest the importance of early oseltamivir treatment. Only one (1%) of 104 patients who tested positive for 2009 pandemic influenza A (H1N1) case-patients died. These findings contrast those of the National Institute of Respiratory Diseases in Mexico City where 12 (67%) of 18 patients required mechanical ventilation and 7 (39%) patients died [24, 26] . The discrepancy between these two case-series may be explained by when the populations served by these hospitals were affected by the pandemic. National Institute of Respiratory Diseases data were collected during March 24-April 24, 2009, when it was still unclear that a proportion of cases with severe acute respiratory infections had 2009 pandemic influenza A (H1N1). In Guadalajara, the outbreak started later. Hospitalized patients we described received earlier oseltamivir. Our patients were hospitalized during April 25-August 9. Seventy-five percent of our case-patients received oseltamivir within 72 hours of symptom onset. Patients in the Mexico City case-series presented with severe disease an average of 8 days after illness onset and received late oseltamivir.