Breast cancer represents 27% of the cancers and 19% of the cancer deaths in female population. The aim of this study was to document the age pattern of the incidence of breast cancer in Iranian female population in the northwest region of the country. The study subjects were 1764 patients with breast cancer diagnosed/registered in the six university clinics between 1988 and 2008 in the northwest of Iran. The highest occurrence rates were observed for the birth year cohorts 1940-1949 (for 59-69 years old), 1950-1959 (for 49-58 years old), and 1960-1969 (for 39-48 years old). Among these three cohorts, the highest rate was observed in 1950-1959 birth year cohort (284.38 per 100,000 female populations, 95% CI: 260-310). This rate was significantly higher compared with the similar rates of other birth cohorts. There was no statistically significant difference between various years in terms of the average age at the diagnosis of breast cancer in our study setting. Despite the previous research reports, we found no significant difference between the mean ages at diagnosis of breast cancer from 1988 to 2008 in Iranian female population.