Asthma is the most common chronic illness in children. It represents a major public health problem worldwide. Being the first study on asthma in Fianarantsoa, the aim of the study is to determine the prevalence, assess the severity and determine the risk factors of adolescent asthma in the city of Fianarantsoa. A prospective cross-sectional study, based on written questionnaires from the ISAAC I survey, among school children aged 13 and 14, over a period of 6 months from February to July 2017 was carried out. The choice of establishment was made randomly. Sociodemographic data, asthma symptoms, other atopic conditions, and students' lifestyle were collected using a standardized questionnaire. Nine hundred and thirty students agreed to participate, giving a participation rate of 81%. Five hundred and fifty-four (59.19%) were girls and 382 (40.81%) were boys. The sex ratio was 0.6. The lifetime prevalence of asthma was 13.26% and asthma in the past 12 months was 3.31%. The wheezing during or after exercise and nocturnal cough during the last 12 months were found respectively in 7.48% and 21.7% of cases. Concerning the severity of asthma, 32.26% presented 4-12 times of wheezing during the last 12 months and 19.35% more than 12 times. Waking up at night more than once a week due to wheezing was found in 25.81% of cases. For the severity of the asthma attack, 35.48% had difficulty speaking, 77.96% absenteeism from school for more than 1 to 5 days. Hospitalization for asthma attack once or twice in the last 12 months was found in 16.12%. The factors associated with asthma and asthma symptoms found were gender (p=0.022), passive smoking (p=0.041), presence of humidity or mold (p=0.003), use of biofuel (wood or charcoal). ) (p=0.017), familial asthma (0.0001) presence of cockroaches at home (0.025). Asthma is relatively common among adolescents in schools in the city of Fianarantsoa. The rate of severe asthma is high. According to our study, many factors are associated with asthma in adolescents.