To analyze the effect of a cell text message reminder service on participation in a mammogram screening program in Catalonia, Spain. A quasi-experimental design was used with women aged 50 to 69 years who had been scheduled mammogram appointments in June or July 2011. Women were personally invited by letter to attend to the breast cancer screening program (n = 12,786). Prior to the invitation, 3,719 (29.1 %) of them had provided their cell telephone number to the National Health Service. These women received a text message reminder 3 days before their scheduled appointment. Logistic regression models were used to analyze whether the text message reminder was associated with participation in screening. Cost-effectiveness of adding a text message reminder to the invitation letter was also analyzed. The overall rate of participation in breast cancer screening was 68.4 %. The participation rate was significantly higher in the text messaging group, with an age-adjusted OR of 1.56 (95 %CI: 1.43-1.70). A detailed analysis showed that the increase in participation related to the text message reminder was higher among women without previous screening who lived in areas where access to postal mail was limited (OR=2.85; 95 %CI: 2.31-3.53) compared to those who lived in areas of easier postal mail access (OR=1.66; 95 %CI: 1.36-2.02). The invitation letter+text message reminder was a cost-effective strategy. Text message reminders are an efficient cost-effective approach to improve participation in difficult-to-reach populations, such as rural areas and newly developed suburbs.