This study investigates the effect of an overt verbal commitment on compliance with returning symptom report cards. Patients in two flu inoculation clinics were placed in an experimental condition. Patients in two other comparable flu inoculation clinics were placed in the control condition. After receiving their inoculations, all patients were asked to mail back a postcard after 48 hours that indicated the occurance of any symptoms. Patients in the experimental condition were asked if they would comply with the request. Patients in the control condition were not asked for a commitment. Patients in the experimental condition returned significantly more cards and significantly sooner that those in the control condition. This technique has both clinical and research applications.