A focused history and a careful physical examination of a hypotonic infant distinguish hypotonia from weakness, essential to diagnosis and treatment. The differential diagnosis of infantile hypotonia can be generated by first determining whether hypotonia is associated with weakness. Global central nervous system (CNS) disorders are suggested by the loss of developmental milestones or evidence of mental retardation. The disorders associated with significant weakness and secondary hypotonia are: spinal muscular atrophy or Werdnig-Hoffmann syndrome, congenital muscular dystrophies, congenital myotonic dystrophy and Guillain-Barré syndrome. Hypotonia with weakness requires the use of electrophysiology and biopsy studies, interpreted in the context of the infant's age. Hospitalization is commonly required to observe a hypotonic infant and to continue with diagnostic evaluation. A pediatric neurologist is consulted to determine the need for further diagnostic procedures or to initiate treatment of an underlying neurological disorder.