From randomly selected patients of doctors affiliated with Beth Israel Deaconess Medical Center in Boston, 48 patients took the history twice with intervals between sessions ranging from one to 35 days (mean seven days; median five days). When we analyzed the inconsistency between first and second interviews with which the 48 patients responded to each of the primary questions. We found that the 215 questions with response options of "Yes," "No," "Uncertain," "Don't understand," and "I'd rather not answer" had the lowest incidence (6 percent); the 10 other multiple choice questions with one response permitted had a 13 percent incidence, and the five multiple choice questions with more than one response permitted had a 14 percent incidence. Whenever an inconsistency was detected with the repeat interview, the patient was asked to choose when appropriate from four possible reasons. Reasons chosen were: "clicked on the wrong choice" (23 percent), "not sure about the answer" (23 percent), "medical situation changed" (6 percent), and "didn't understand the question (less than 1 percent). With the remaining 47 percent of the inconsistencies, no reason was given.