This thesis sought to evaluate the effect of Ontario's Universal Influenza Immunization Program (UIIP) on pneumonia and influenza (P&I) hospitalizations. Weekly P&I hospitalizations from September 1993 to March 2003 in the 10 Canadian provinces were extracted from a national hospital discharge abstract database. The effect of the UIIP was estimated by comparing influenza season-associated P&I hospitalizations before versus after the 2000-01 season for Ontario and for all other provinces which persisted with targeted immunization programs (UIIP effect). Overall and age-specific Poisson regression models were run for each province to generate relative rate (RR) estimates of the UIIP effect. There were 1,185,588 P&I hospitalizations included in the study. UIIP introduction was not associated with an overall decrease in P&I hospitalizations in Ontario compared with other provinces. Relatively greater decreases were observed in those aged 0-49 years in Ontario compared to other provinces, but not in those 50 years or older.