Methylmalonic Acidemia (MMA) is an inborn error of metabolism that results in accumulation of methylmalonic acid in blood and increased excretion in urine. The effects of MMA vary from mild to life threatening and it usually presents in early infancy. Affected infants can have vomiting, dehydration, hypotonia, developmental delay and failure to thrive. The emergency treatment of the newborn with MMA mainly comprises rehydration and promotion of anabolism, followed by long-term dietary management by both the restriction of precursor amino acids using a low protein diet and avoidance of prolonged fasting. Prognosis depends on the type of MMA and whether the condition is well controlled in general and during episodes of metabolic decompensation. We report here the presentation and management of a 2-year boy with MMA who failed to achieve expected milestones for age. To the best of our knowledge, only one case of MMA has been reported from Pakistan.