The level of knowledge was established on Sexually Transmitted Infection (STI) before andafter the application of an educational workshop and the principal source of information amonghigh school students of the Lyceum Bolivariano "Rafael Monasterios" of Barquisimeto, in thelast four-month of 2007, with a study of quasi-experimental type, with accidental sampling of48 students of a population of 237, applying a survey constructed with closed questions ofsimple selection. Obtaining that 8,33% of the participants corresponded to the satisfactorycategory before the workshop and later to the same one it increased to 62,50%. As for thechangeable age appreciated that the level of knowledge increased considerably in all the groupsafter the application of the workshop. With regard to the sex, both in the feminine and in themasculine, the percentage of the participants who were located in the satisfactory categoryincreased notably later to the workshop, being more evident in the feminine sex. On havingevaluated the topics on generalities, mechanisms of transmission and methods of prevention ofthe STI, one thought that before the workshop the level of knowledge was satisfactory in56,25%, 12,50% and 25% respectively and after the same one the percentages obtained for thesame category were 93,75%, 66,67% and 60,41%. The parents constituted the principal sourceof information, followed by the teachers. This study constitutes a contribution to provide basesthat they contribute to the strengthening of the educational existing programs on the STIdirected the teenagers.