Human immuno-deficiency virus (HIV) infection is one of the leading public health problems in our country. Preventive education has shown itself to be a useful tool for curtailing this disease. The objective of this study is that of assessing the effectiveness of a preventive intervention at the Secondary Schools in the Baix Ebre and Montsià areas of the Tortosa Healthcare District aimed at reducing the risk of HIV transmission and furthering the knowledge of this disease. A pre/post intervention study was conducted with no control group. A total of 19 schools, corresponding to the fourth year of Compulsory Secondary Education and first year of "Bachillerato" studies were invited to participate. The activity consisted of a workshop 60-90 minutes in length, in conjunction with educational material. A questionnaire was used as the gauging tool. Twelve schools agreed to participate, a total of 896 answers having been gotten for the pre-test and 805 answers for the post-test. A statistically significant increase was found for 52% (10/19) of the items regarding the correct answers on the post-test. A statistically significant improvements was found in four of the five items (80%) related to attitudes, and in five of the eight (62.5%) items related to overall knowledge of this infection following the intervention. A statistically significant improvement regarding prevention-related knowledge was found solely in one of the six items (16.6%). Preventive activities in the form of participational workshops are a good way of furthering the knowledge and improving the attitudes regarding HIV/AIDS of the participating teenagers. As no curative treatment exists for this disease, prevention from the educational realm is of importance for curtailing increased HIV transmission.