The aim of this study was to determine the best upper age limit for a breast cancer screening programme. We used a model-based study using optimistic and pessimistic assumptions, concerning improvement of prognosis due to screen-detection and duration of the period of mammographic detectability, resulting in upper and lower limits for favourable and unfavourable effects. Under pessimistic assumptions, the balance between positive and negative effects of screening remains favourable up to an age of around 80 years. Under optimistic assumptions, this balance never becomes clearly negative with increase of the upper age limit of a screening programme. When including the costs in the analysis, the balance between effects and costs of increasing the upper age limit from 69 to 75 years is likely to be at least as favourable as intensifying a screening programme within the age group 50-69 years. A further increase leads to a markedly less favourable balance. Competing causes of death do not lead to missing net benefit for women up to at least age 80 years, but the disproportional rise of negative effects of screening with age in older women leads to a lower cost-effectiveness ratio than intensifying screening at ages 50-69 years.