BackgroundAdolescence is a life spanning the age range of 10-19 years characterized by remarkable physical and psychological growth and development. One-third of adult weight and more than one-fifth of adult height is gained during this period of life. This make adolescents the key population group highly vulnerable to malnutrition. This study was aimed to assess the level of undernutrition among school adolescents in Wonago district, Gedeo Zone, South Ethiopia.MethodsA school based cross sectional study was conducted among 443 randomly selected school adolescents of Wonago district. Nutritional status of adolescents was assessed using body mass index for age z-score (BAZ) and height for age z-score. Descriptively, prevalence of thinness and other anthropometric measurements, socio-economic and socio-demographic variables were described. Multivariable logistic regression was conducted to assess the factors associated with adolescents’ undernutrition (thinness).ResultsThe finding revealed that, 11.6% of the adolescents were undernourished (thin). The likelihood of being undernourished was decreased by 70% for female adolescents, and adolescents whom the source of drinking water was spring were four times at higher risk of malnutrition. The likelihood of being undernourished was decreased by about 85% for adolescents from households with medium wealth status. Adolescents from families achieving food needed for daily consumption through purchasing and adolescents whose the occupation of their father was merchant were also found to have a higher risk of being undernourished. For adolescents whose the occupation of their mother was farmer, the probability of being undernourished was decreased by about 85%.ConclusionsSignificant number of adolescents in this study were found to be malnourished. Sex of the adolescents, parent occupation, household wealth, source of drinking water and source of family food need were significantly associated with the adolescents’ nutritional status.