The age-standardized lung cancer incidence rate among women in the United States has decreased for each of the last 3 years for which data are available (1999-2001). We conducted this study to assess the stability and near-term sustainability of this decrease. We examined temporal trends in age-specific lung cancer incidence by calendar year and birth cohort and measured trends in the age-standardized rate in each geographic area within the Surveillance, Epidemiology, and End Results (SEER) Program using joinpoint regression analyses. Age-standardized lung cancer incidence rates have peaked or are decreasing in all geographic areas within SEER, although the decline is statistically significant only in San Francisco-Oakland. Age-specific incidence rates are decreasing in six of the seven 5-year age groups between ages 50 and 84 years in all areas of SEER combined. Rates in these age groups contribute nearly 95% of the total age-standardized incidence rate; consequently, trends in incidence at these ages will determine future trends in the overall age-standardized incidence rate for the next 20 to 25 years. Birth cohort patterns suggest that the decrease in the age-standardized rate will continue for at least 20 years, but will be slowed by aging of women born in the late 1950s and early 1960s. Given calendar year and birth cohort age-specific incidence patterns, the early decline in lung cancer incidence among women is likely to persist through at least 2025. Sustaining the downward trend beyond 2025 will require continued reductions in smoking initiation among children and increases in cessation among addicted smokers.