The objective of the study was to assess the effect of an invitation letter on the level of participation in a setting of mainly opportunistic screening for cervical cancer and to do a cost analysis of this intervention. We designed a quasi-randomized trial in which a sample of women between the ages of 25 and 64 years and residing in the province of Limburg, Belgium, who had no Pap smear taken in the past 30 months according to LIKAR (Limburg Cancer Registry), were assigned to an intervention group or to a control group. A written invitation was sent to 43 523 women in the intervention group. Baseline participation in cervical screening was recorded in the year before the intervention to determine its effect. Differences in cumulative incidence between the intervention and the control group were used to report the effect. The net effect of a written invitation resulted in 3355 more women undergoing a Pap smear, which is an increase of 6.4% (95% confidence interval: 5.9–6.9). The cost per additional Pap smear taken amounted to €29.8. Within an opportunistic cervical cancer screening setting, the effect of a registry-based invitational programme to nonattenders increases the participation further, and at no extra cost compared with an invitational programme to all screen-eligible women irrespective of their screening status.