Aim: The Prevalence of Thinness and Associated Risk Factors among School Going Adolescents. Materials and methods: This was a cross-sectional study conducted in the Department of Home Science, Purnea University, Purnea, Bihar, India, for 1 year. The study was conducted in 3 secondary school in Purnea, Bihar. This study was conducted among adolescents aged between 12 and 18 years attending secondary school in Purnea, Bihar. A simple random sampling technique was used to pick the five schools and a Systematic sampling was used to obtain the number of children from each school. A sampling frame derived from a list of pupils aged between 12-18 years in the randomly selected schools was then used. Results: A total of 100 adolescent girls were included in the study with a response rate of 100%. The mean age of respondentswas 16.17 (±2.77 standard deviation) years. Nearly one-quarter (25%) of respondents was found to be in thefirst quintile range of wealth index. Majority 90(90%)ofthe respondents were followers of Hindu,followed by Muslims (8%). 65(65%) of the respondents were urban dwellers.Regarding parents of the study participants 30 (30%) of mothers and 36 (36%) of fathers were educated at least at a primary school level. By occupation majority 72(72%) of mothers and 40(40%) of fathers were housewives and farmers, respectively. From the respondents, 55(55%) and 55(55%) were involved in moderate to vigorous work and moderate intensity sport activities for ≥10 min continuously per day, respectively. Regarding to eating habit of the respondents, 22(22%), 20 (20%), and 65 (65%) of them ate fruits, vegetables, and snack, respectively, at least once/day. In general, only 5(5%) of the respondents consumed at least three meals/day. 88 (88%) of therespondents had used adequate diet diversity and the rest had used inadequate diet diversity. Of the respondents, 45(45%) had begun their first menstruation in the agerange of 14–16 years of age and 5 (5%) had begun their first menstruation above 16 years of age. Conclusion Thinness was found to be a serious problem in the study area. Strategies are needed to improvethe nutritional status of adolescent girls before they reach conception to break the vicious cycle of intergenerational malnutrition.