Hypotonia involves decreased muscle tone. Infants with hypotonia seem floppy and feel like a "rag doll" does when held.

Hypotonia is often a sign of a worrisome abnormality and may suggest the presence of central nervous system dysfunction, genetic disorders, or muscle disorders.

Hypotonic infants rest with their elbows and knees loosely extended, while infants with normal tone tend to have flexed elbows and knees. Head control may be poor or absent in the floppy infant with the head falling to the side, backward, or forward.

Infants with normal tone can be lifted with the parent's hands placed under the armpits, while hypotonic infants tend to slip between the hands as the infant's arms rise unresistingly upward.

• Down syndrome
• Myasthenia gravis
• Prader-Willi syndrome
• Kernicterus (brain damage due to severe jaundice; causes can include Rh incompatibility)
• Cerebellar ataxia, congenital
• Myotonic dystrophy (a type of muscular dystrophy)
• Infant botulism
• Familial dysautonomia (Riley-Day syndrome)
• Marfan's syndrome
• Muscular dystrophy
• Achondroplasia
• Trisomy 13
• Sepsis
• Aicardi syndrome
• Canavan disease
• Congenital hypothyroidism
• Hypervitaminosis
• Krabbe disease
• Menkes syndrome
• Metachromatic leukodystrophy
• Methylmalonic acidemia
• Rickets
• Spinal muscular atrophy type 1 (Werdnig- Hoffman)
• Tay-Sachs disease
• Vaccine reaction
• Klinefelter syndrome
• Phelen-McDermid syndrome (Deletion 22q13 syndrome)

Extra care must be taken when lifting and carrying a hypotonic infant to avoid causing an injury to the child.