Sirs—In their study on women's perception of the benefits of mammography screening in four countries, Domenighetti and colleagues found an overestimation of the benefits of mammography screening. 1 While every effort to tackle the difficult but essential issue of balanced risk information in health prevention is commendable, we have serious concern about two aspects of this study, i.e. its design and conclusions. Two of the three questions on which the study is based addressed, respectively, the perceived relative (Q2) and absolute (Q3) benefits of regular mammography screening. As the wording of both questions suggests that mammography screening reduces breast cancer mortality and the correct answer is systematically the smallest positive effect among the proposed answers, underestimation of the quantification of the beneficial effect of mammography screening is virtually impossible in this study. 2 Further, the restriction to one correct or most appropriate answer for Q2 and Q3 is also questionable. Reporting of the impact of regular mammography screening on breast cancer mortality ranges from none 3,4 to about a 50% reduction. 5,6 When, as is the case for mammography screening, disseminated messages about the effect are heterogeneous, reflecting in part the diverging opinions held by health professionals on the issue, it is not surprising to observe comparable variations in the perceived quantification of the benefits among the female population. The complexity of Q3 may furthermore require a particularly astute mind, from unprepared respondents, to work out the mathematics behind it (trying this question on work colleagues is informative in this respect). Question 1, which highlights what screening cannot achieve (primary prevention), with a misleading statement, further emphasizes the crucial importance of wording and selection of answers for closed questions, and the distorting effect that apriorism can have on a study design. 2 In the Swiss canton of Vaud where organized screening has been offered to 50-69 year old women for a decade, 7 the question 'What is the purpose of mammography screening?' was asked in two consecutive random phone surveys of 50-69 year old females, with different allowed answers. One objective was to assess the possible confusion around the term prevention which was believed to be understood by some as 'prevents the development of a breast cancer toward a fatal outcome'. When 'to prevent cancer' was a proposed answer, a majority of respondents opted for this choice with only 43% of females agreeing that mammography screening enables detection of a lesion and the offer of a less- aggressive treatment (data available on request). However, 93% of respondents adequately stated that screening enables detection of a breast anomaly when 'to prevent cancer' was replaced by the probably less confusing 'to avoid cancer' (4% of respondents elicited this answer) in the second survey conducted about 4 months later. Adequate quantification of current misconceptions about screening 8 is necessary to assess the effectiveness of future strategies aimed at improving public understanding.