Studies of the effectiveness of telephone reminders to improve the rate of appointments kept have shown conflicting results. Few have accounted for patients without telephones. We conducted a controlled clinical trial to study the effect of excluding patients without telephones in a study of telephone reminders in an urban pediatric clinic. Telephone reminders were attempted the evening before scheduled visits for 238 subjects; 259 controls received no reminders. The overall rate of kept appointments was the same (54%) for both the intervention group and the control group. In neither group was there a substantial difference in the "show" rate between patients with and without telephones (intervention group 52% versus 57%, control group 55% versus 54%). In the intervention group, the show rate was higher for those with telephones who were contacted (66%) than for those who were not (37%) (P < .001). Telephone reminders did not improve the overall rate of kept appointments but appeared to result in a significant improvement in the show rate of those who could be reached. Excluding patients who could not be reached by telephone may affect the usefulness of previous studies.