This study assessed the impact of automated telephone reminders in a population of 2,008 patients scheduled for appointments in a public health tuberculosis clinic. Overall, remainders increased appointment attendance from 52% to 62%. Reminders were more effective for some applications than others, but the effectiveness of reminders did not differ significantly across patient age, sex, or ethnicity. Counter to theoretical predictions, neither attribution of the reminder message to an authority nor a statement stressing the importance of the appointment significantly increased the effectiveness of the reminder above the level obtained without these enhancements.