A comprehensive examination of gender and age-specific influenza and pneumonia hospitalization seasonality is currently lacking. Using population-based data for Ontario, Canada between April 1988 and March 2002 (n=339803 hospitalizations), findings from this study revealed clear seasonality [Fisher's Kappa (FK) test=68·64, P<0·001; Bartlett's Kolmogorov–Smirnov (BKS) test=0·68, P<0·001] with consistent summer troughs and winter peaks for both sexes and all ages combined. The very young (both sexes 0–4 years) demonstrated the strongest seasonality (Rautoreg2=0·97) and females aged 10–19 years, the weakest (Rautoreg2=0·59). Gender differences were most pronounced in the oldest age groups (80+ years) where females had an average annualized peak rate of 250/100000 compared to 400/100000 for males. These findings can contribute to more population-specific prevention strategies and effective resource and service allocation based on seasonal and specific population demands.