Methylmalonic Acidemia (MMA) is a group of inborn errors of metabolism (IEMs), specifically of propionate catabolism characterized by gastrointestinal and neurometabolic manifestations resulting from a deficiency in the function of methyl malonyl-CoA mutase, methylmalonyl-CoA epimerase, and cobalamin metabolism. Aim: To investigate the effectiveness of instructional guidelines implementation on mothers' knowledge and practice regarding their children with methylmalonic academia. Design: A quasi-experimental design was used to achieve the aim of this study. Setting: This study was conducted at the Specialized Genetic Outpatient Clinic at Sohag University Hospital. Subject: A purposive sample of 50 mothers and their children with methylmalonic academia was selected from the previously selected settings. Three tools were used: (1) a structured interviewing questionnaire, (2) Mothers’ knowledge regarding methylmalonic academia, and (3) Mothers’ Reported Practices format regarding methylmalonic academia. Results: The current study results revealed that two-quarters of the mothers had unsatisfactory knowledge about methylmalonic acidemia pre-instructional guidelines implementation compared with the majority of them reported satisfactory total knowledge post-instructional guidelines implementation; As well, mothers' total reported practices were significantly improved post-instructional guidelines implementation.