To describe the sex and age differences in asthma hospitalization among the Canadian population, we conducted an analysis based on a total of 9,486,173 hospital records in Canada for a 3-year period (1994/1995, 1995/1996, and 1996/1997), including 204,304 asthma patients and 288,977 asthma-related records. Asthma as one of the first five diagnoses, accounted for 3.0% of total hospitalizations, which was almost constant across the 3-year study period. The 3-year cumulative incidence of asthma hospitalization was substantially higher for young boys than girls, and it was reversed for adults. The incidence ratio for females vs. males for asthma hospitalization reached 2.8 for individuals 25 to 34 years of age, decreased gradually with increasing age, and then approached unity for those aged 80 years or more. The data suggest that sex is an important determinant for asthma, and the sex effect varies considerably over a life span.