Nearly 35% of the women invited to the gynaecological mass screening in Uppsala do not participate. It is therefore of importance to study the reasons for this, especially as previous research in this field is sparse. We used a mail questionnaire which included questions concerning sociological data, gynaecological background, attitude to gynaecological examinations and to mass screening, attitude to cancer and to sexual matters. 87.5% of the sample of 800 women replied. We compared the group of women who had at some time participated in the mass screening with those who had never participated. The sociological variables were found to be closely correlated, and their individual significance could consequently not be distinguished. Those who had not participated did not as a whole constitute a critical group as regards detection of cancer, since it was found that most of these women had attended a gynaecologist elsewhere and, although they had had smears taken, they appeared to be poorly informed as to the type of cancer that the smear is intended to detect. Our questioning failed to show the women's attitude to cancer or to sexual matters was of significance for participation, although there is some evidence that the latter may be of some importance. The most significant difference disclosed between the groups was that non-participants consider gynaecological examinations more unpleasant than do participants. Furthermore it was found that women's expectations of the mass screening differ from what they consider to be included in the examination. Finally, measures intended to increase participation are suggested.