OBJECTIVES: We examined the frequency of mammography screening among women who had had a screening mammogram recently and therefore generally did not need to repeat the examination. METHODS: A population-based sample of 50- to 69-year-old women were surveyed immediately before and 8 months after they received an invitation to participate in the first round of screening of the newly organized mammography screening programme in Geneva, Switzerland. These women also received a booklet that included the recommendation to have screening mammograms at 2-year intervals. RESULTS: The baseline survey identified 660 women who had had a mammogram within the previous 12 months. Of these, 23.2% [95% confidence interval (CI), 20.0-26.6] had an opportunistic mammogram and 4.1% (95% CI, 2.7-5.9) had an organized mammogram during follow-up. Women who had had their last mammogram 6-12 months prior to baseline (vs. more recently), intended to have a mammogram within the next 6 months, wished to receive more information on mammography screening, and had a history of surgical breast biopsy were more likely to have an unnecessary screening mammogram (either organized or opportunistic) during follow-up. Compared with women who had an opportunistic mammogram, women who had an organized mammogram were more likely to be of lower socioeconomic status, to have made their own screening decision and to have anticipated the date of their next mammogram by no more than a few months. CONCLUSIONS: Opportunistic mammography screening in excess of recommendation is common, and persists despite explicit advice about recommended screening frequency.