Mucopolysaccharidosis III (MPS III) or Sanfilippo disease is a rare error of Glycosaminoglycan (GAG) metabolism resulting from the absence of any of the four enzymes that metabolise Heparan Sulphate (HS). We report a case of MPS III presenting with recurrent chest infections, global developmental delay, growth failure and nocturnal hyperactivity. A high threshold of suspicion for both the minimal somatic signs as well as typical behavioural changes of MPS III need to be maintained in clinical practice in conjunction with other less common presenting features. The role of anthropometric assessment, ophthalmological assessment, screening of urine for GAG excretion, and neuroimaging to determine extent of neurological involvement and prognosticating severity is described with review of relevant literature. A thorough clinical examination with history taking is important in any child with developmental delays, and organomegaly with dysmorphism can point the finger at a metabolic or storage disorder that can otherwise go unnoticed