The Gang Membership Inventory (GMI) was developed for use with urban high-risk youth and administered as part of a larger survey. The GMI consists of 15 true/false items asking about the respondent's involvement in gang activities in the year just past. Activities range from peripheral involvement to more serious involvement in gangs (e.g., being an actual gang member). The GMI was administered in English or Spanish to fifth through eighth graders at two Chicago (Illinois) elementary schools in predominantly Latino neighborhoods. Usable surveys were obtained from approximately 650 children. Forty percent of the respondents self-identified themselves as Puerto Rican, 28 percent as Mexican, 5 percent as both Mexican and Puerto Rican, and 7 percent as other Latino ancestry. Principal components analysis with varimax rotation produced three factors with eigenvalues greater than 1.0 that together accounted for 50 percent of the total variance. Because the factors do not appear to be well-defined, it was decided to retain the measure as a single scale. Validity of the GMI was assessed through a one-way analysis of variance and other measures. Also examined was the relationship between gang involvement and the subscales of the Teacher-Child Rating Scale. As expected, youth rated high or moderate on acting-out behavior were significantly more gang involved than were youth low in acting out. The GMI has been shown to be easy to administer and useful in determining different levels of involvement in youth gangs. One table displays GMI factor loadings, and six figures describe relationships of various variables to gang involvement. An appendix contains the GMI. (SLD)