Asthma is one of the most prevalent chronic diseases worldwide, with a considerable increase, especially in children. It is considered the main cause of childhood morbidity, school absenteeism, and limitations in sports practice. The causes are multifactorial, and their prevalence varies from region to region, thus verifying a great disparity in the estimates of the prevalence of asthma. In this sense, the objective of this study is to investigate the prevalence of asthma, its control, as well as the frequency of associated symptoms, in adolescents who attended the 3rd cycle of basic education and secondary education in schools in the municipalities of Paços de Ferreira, Paredes, and Penafiel. The sample consisted of 1222 (587 males and 635 females) (p = 0.17) aged between 12 and 17 years. The instruments used to diagnose asthma-associated symptoms were the standard questionnaire of the “International Study of Asthma and Allergies in Childhood—ISAAC” and to check whether asthma was controlled, the “Test for Asthma Control” questionnaire was used. The results reveal a high prevalence of adolescents with asthma (8.9%) with a significant percentage that did not have the disease under control (38%). There was also a considerable percentage of adolescents who, despite not having asthma, have many symptoms associated with the disease. These results may be associated with environmental factors.