The purpose of this instrumental research was to expand the Internalized Homonegativity Scale to assess a new factor, the fear of being identified as a non-heterosexual person, and to validate the new instrument. A questionnaire was applied to an incidental sample of 200 medical students from a private university in Monterrey, Mexico. This questionnaire was composed of four items about sociodemographic information, one is about self-defined sexual orientation, 16-item Internalized Homonegativity Scale (plus five items to assess the new factor), Attitude Towards Lesbians and Gay Men Scale, and Kinsey's Sexual Orientation Scale. The overall internal consistency was excellent. Upon removing two items, a four-factor model was reproduced through exploratory factor analysis; the goodness of fit was good. The four factors showed evidence of internal consistency and discriminant validity. The IHN-21 total score followed a normal distribution and its mean was higher among men than among women. IHN-21 had negative correlation with age and sexual orientation, and positive correlation with attitude of rejection towards gay men and lesbians. It is concluded that the scale shows internal consistency and reliability and construct validity.