Objectives: We sought to determine the prevalence of inborn errors of metabolism (IEM) in children with unexplained developmental delay and their types. Methods We conducted a cross-sectional study in Misan, Iraq, over a period of one year. A total of 112 infants with unexplained developmental delay were included in the study, and the required information was taken from their parents by direct interview. Tandem mass spectrometry (MS/MS) was done in collaboration with MedLabs' Referral Laboratory in Amman, Jordan. Results: Twenty (17.9%) cases had abnormal MS/MS. Disorders of amino acid metabolism represented the majority of IEM (10 cases) in which phenylketonuria and maple syrup urine disease were the most common (found in five cases each). Organic and fatty acid metabolisms were found in five and two cases, respectively. Most cases of IEM had a positive family history and consanguinity, however, family history was the only significant factor (p < 0.001). Conclusions: A high rate of IEM was detected in children with unexplained developmental delay in Misan. A high clinical suspicion with positive family history and consanguinity supported by the MS/MS results played an essential role in the diagnosis. However, implementation of newborn screening is essential for early diagnosis and to determine appropriate therapy in newborns with IEM in Iraq generally and in Misan in particular.