The authors have made a research on the influence of mailing letters on keeping up the returns in an ambulatory of health care service. The purpose of the paper was to seek a way to improve the return rate, which is usually low, and brings up difficulties to structure adequately a staff. One hundred children who had failed to come to their last appointment were studied and divided in two groups: 50 children to whose responsible were sent letters setting up a new appointment were included in group I; group II served as a control group. Out of the 97 absentees, 36 children (37.1%) returned. Their return was spontaneous in 22 cases (22.7%)--12 children of group I and 10 of group II. 14 mothers returned on the date set up by mail. Out of the 47 mailed letters, 20 (42.5%) were sent back by the post office. The authors have concluded that mailing letters was an effective method to decrease the number of children who miss their follow-up in ambulatory of health care service.