Bullying against lesbian and gay adolescents continues to be a problem in Western societies. Interventions to reduce homonegative attitudes could help to address bullying. We evaluated the effectiveness of a workshop against homonegativity in adolescents (N = 214) aged between 14 and 16 years in a pretest, posttest, follow-up design with a control group who participated in this workshop afterwards. Results revealed that the workshop reduced homonegativity in adolescents directly after the workshop, but not 6 weeks after the workshop. Moreover, we investigated predictors of the effectiveness of the workshop. Regression analyses revealed that the level of homonegativity 1 week prior to the workshop, gender role orientation, and religious denomination explained 28.6% of the variance in the reduction of homonegativity directly after the workshop. Based on the results, the effectiveness of one-time workshops is discussed.