Background:Menstrual irregularity is a common gynecological problem among reproductivernage women affecting not only their social activities and school attendance but also families, rnsocial and national economy as well. This problem for female students in developing countriesrnwho may face life-threatening condition is considered as minor health concern. Inconsistent andrninconclusive findings on menstrual irregularity and its associated factors were indicated.rnMoreover, data on magnitude and factors associated with menstrual irregularity is scarce inrnEthiopia. rnObjectives:The aim of this study was to assess prevalence of menstrual irregularity andrnassociated factors among preparatory school girls. rn rnMethods:Institution-based, cross-sectional study was conducted among 367 female students inrnfive selected schools from Nov. 20 to Dec.10, 2020, in Adama Town of Oromia Regional State rnin Ethiopia. Stratified random sampling technique was used to select study participant from bothrnpublic and private schools. Data were collected using a pre-tested, structured self-administeredrnquestionnaire. Data were entered and analysed using SPSS version 25. Descriptive analysis wasrnperformed using frequency, percentage, mean and standard deviation. Chi-square test was usedrnto determine the relationship between menstrual irregularity and categorical variables.rnFurthermore multivariable binary logistic regression was used to test the presence of associationrnbetween Menstrual irregularity and all other independent variables like age at menarche, stress,rnBMI and sleep duration. Statistical significance was considered at P