The frequency, prevalence, and mortality patterns of cystic fibrosis were analyzed in 3,795 patients documented in the Canadian Patient Data Registry in 1970-1989. Cystic fibrosis frequency in the 1970-1979 birth cohort was virtually identical to the commonly quoted 1 in 2,500. In 1985-1989, median survival age was 36.7 years for males and 27.8 years for females, compared with 26.6 and 19.7, respectively, in 1970-1974. However, there were significant regional differences when Canada was divided into the four regions, East, Quebec, Ontario, and West. In Quebec, patients were younger at diagnosis and until recently had greater mortality than patients in other regions, which suggests more severe disease; dramatically improved survival in the 1980s coincided with a change from a restricted fat diet to a high fat diet. Improved survival in Ontario in the 1970s accompanied this change in dietary therapy, which may also account for good survival throughout the study period in the East. The West showed gradually improving survival, similar to that reported in other parts of the world. Proportional hazards analysis showed pulmonary function to be the best predictor of survival. Poorer survival in females was associated with poorer weight, but the interrelation of declining pulmonary function, weight maintenance, sex, and mortality requires further study. The effect of pulmonary colonization with Pseudomonas aeruginosa was confounded with degree of pulmonary dysfunction, but colonization with Burkholderia cepacia (previously Pseudomonas cepacia) was associated with increased mortality at all levels of pulmonary function.