['Evidence regarding the mortality rate after administration of the pandemic influenza A (H1N1) 2009 vaccine on patients with underlying diseases is currently scarce. We conducted a case-control study in Japan to compare the mortality rates of patients with idiopathic interstitial pneumonia after the vaccines were administered and were not administered.', 'Between October 2009 and March 2010, we collected clinical records in Japan and conducted a 1:1 matched case-control study. Patients with idiopathic interstitial pneumonia who died during this period were considered case patients, and those who survived were considered control patients. We determined and compared the proportion of each group that received the pandemic influenza A (H1N1) 2009 vaccine and estimated the odds ratio. Finally, we conducted simulations that compensated for the shortcomings of the study associated with adjusted severity of idiopathic interstitial pneumonia.', 'The case and control groups each comprised of 75 patients with idiopathic interstitial pneumonia. The proportion of patients who received the pandemic influenza A (H1N1) 2009 vaccine was 30.7% and 38.7% for the case and control groups, respectively. During that winter, the crude conditional odds ratio of mortality was 0.63 (95% confidence interval, 0.25-1.47) and the adjusted conditional odds ratio was 1.18 (95% confidence interval, 0.33-4.49); neither was significant. The simulation study showed more accurate conditional odds ratios of 0.63-0.71.'] Does the pandemic influenza A ( H1N1 ) 2009 vaccine increase the mortality rate of idiopathic interstitial pneumonia : a matched case-control study?