Objective: Because physicians customarily obtain histories about possible sexual abuse before examining children, and because their opinions can influence social and legal outcomes, we investigated whether girls' histories influence physicians' interpretations of their genital findings. Design: In mailed questionnaires, 1,387 randomly selected Fellows of the American Academy of Pediatrics and all 802 members of 4 professional groups concerned with child abuse or pediatric gynecology were asked to interpret 7 simulated cases, each containing a brief history and a photo of a girl's external genitalia Respondents were asked to interpret 7 more cases in a second questionnaire mailed 4 months later. Both sets of cases involved the identical 7 photos. In 6 case pairs, the 2 histories linked to the photo differed in the extent to which they suggested sexual abuse. In the seventh pair--a control--the identical history was presented in both cases.Results: The response rate was 51%. Data from 28% of 2,189 potential respondents were analyzable. Mean interpretation scores for genital findings changed when the histories changed in 2 case pairs for the 110 experienced (Exp) physicians, in 5 pairs for the 278 moderately experienced(Modexp) physicians, and in all 6 pairs for the 216 inexperienced physicians(Inexp) physicians (P≤0.01 for each case pair). The proportion of Exp physicians who reversed their interpretations of genital findings in the direction suggested by the history--from “no indication of abuse” to “probable abuse” or vice versa--ranged from 0%-6% in the 6 case pairs with differing histories. The comparable proportions were 2%-20% for Modexp physicians and 4%-27% for Inexp physicians. In 4 of 6 cases, Exp physicians reversed their interpretations of genital findings less often than did the less experienced physicians (P≤0.02). The Exp and Modexp physicians interpreted 2 of the 3 case pairs with unambiguous genital findings more consistently than they interpreted the control pair (P≤0.02).Conclusions: In 6 simulated cases, diagnostic expectation influenced some physicians' interpretations of girls' genital findings. Physicians should be alert to the likelihood of this bias and its potentially serious social and legal consequences.