The motives and reasons for regular attendance, irregular attendance and drop-out were studied in women who were enrolled in a biennial breast screening programme in 1975 and who were invited to each subsequent screening round until 1992. Three compliance groups were compared: 'attended all rounds' (group A, n=79), 'missed 1 or 2 rounds' (group B, n=73) and 'missed more than 2 rounds' (group C, n=64). The groups did not differ with respect to background variables such as sociodemographic characteristics, actual health problems or preventive health orientations, but significant differences were found in general attitudes to breast screening and to the organizational aspects of screening procedures. The results suggest that during the course of a screening programme, for a substantial group of not strongly motivated women, the recurring negative aspects of mammography (pain and anxiety) are increasingly becoming a burden. 'Circumstantial factors' like waiting for one's turn, the distance to the screening centre and incidental dissatisfaction with handling by screening staff, appear to trigger the decision to skip screening rounds or to drop out of the programme. From the perspective of maintaining a regular attendance throughout the programme this is an important group. Special efforts must be made to encourage these women to stay in the programme.