Project Abstract Inborn errors of metabolism (IEM) are a group of rare genetic (inherited) disorders that cause a block in a metabolic pathway leading to clinically significant consequences, causing developmental and intellectual disabilities, seizures, and, if untreated, coma and death. The majority of the IEM disorders are managed by manipulation and modification of diet alone to reduce toxic metabolites, or with a combination of dietary modification and medications. The complex dietary therapies used in IEM challenge routine dietary recommendations and render diet apps designed for the general public of little utility in these conditions. Also, no diet app is developed specifically for the IEM community and meets the rigor required for research, particularly the need to measure specific and multiple amino acid content in the diet to facilitate more study on impact on brain neurochemistry. Within the field of IEM, paper diet diaries are utilized to monitor food intake. These diaries are frequently inadequate in capturing what is actually being consumed due to recall errors and other factors. Novel methods of assessing dietary intake are required to reduce the diet tracking burden, improve accuracy in dietary surveys, and improve diet adherence. We propose to develop and validate MyRareDiet® (MRD) to address an unmet need in the IEM population to assist with dietary management designed to increase adherence and compliance that could lead to improved outcomes, while facilitating the collection of dietary data from individuals with IEM for research purposes. MRD includes a Patient Portal, a Clinical Dietician Portal, and a Researcher Portal. The Patient Portal is to help IEM patients track and monitor their own diet so they can meet their strict diet targets and restrictions. The Clinical Dietician Portal is to help clinical dieticians manage diet modifications for individuals with IEM. The Researcher Portal is to facilitate the conduct of diet-based studies and the analysis of study results. The aims for Phase II are: 1) enhance and refine the Phase I MRD prototype by adding features to improve diet reporting accuracy and reduce user burden; 2) conduct a criterion validity study to test the agreement between MRD and a widely used and validated interview-based 24-hour recall method; and 3) conduct a randomized controlled trial to assess the effects of using MRD on participants’ diet tracking and monitoring behaviors. Our hypotheses are that compared to paper diet diaries, the use of MRD will result in 1) higher satisfaction with greater adherence to self-reporting of dietary intake; and 2) protein consumption more closely aligned to protein restriction target.