The aim of the study was to analyze, using a decision analysis approach, the probability of severity of illness due to delayed utilization of health services and inappropriate hospital medical treatment during the 2009 AH1N1 influenza epidemic in Mexico. Patients with influenza AH1N1 confirmed by the polymerase chain reaction (PCR) test from two hospitals in Mexico City, were included. Path methodology based upon literature and validated by clinical experts was followed. The probability for severe illness originated from delayed utilization of health services, delayed prescription of neuraminidase inhibitors (NAIs) and inappropriate use of antibiotics was assessed. Ninety-nine patients were analyzed, and 16% developed severe illness. Most patients received NAIs and 85.9% received antibiotics. Inappropriate use of antibiotics was observed in 70.7% of cases. Early utilization of services increased the likelihood of non-severe illness (cumulative probability CP = 0.56). The major cumulative probability for severe illness was observed when prescription of NAIs was delayed (CP = 0.19). Delayed prescription of NAIs and irrational use of antibiotics are critical decisions for unfavorable outcomes in patients suffering influenza AH1N1. Copyright © 2017 IMSS. Published by Elsevier Inc. All rights reserved.