Mosaic trisomy 8 is a chromosomal abnormality that can affect many parts of the body. In individuals with mosaic trisomy 8, some of the body's cells have three copies of chromosome 8 (trisomy), while other cells have the usual two copies of this chromosome. The signs and symptoms vary, but may include distinctive facial features; intellectual disability; and joint, kidney, cardiac, and skeletal abnormalities. Males are more frequently affected than females. In the absence of serious problems, life expectancy is normal, while complete trisomy 8 is lethal and often results in miscarriage during the first trimester. Most cases are not inherited; it often occurs sporadically as a random event during the formation of the reproductive cells (egg and sperm). Diagnosis is based on analysis of chromosomes via genetic testing, such as karyotype. Treatment varies depending on associated symptoms, but typically requires a multidisciplinary team approach.

In other cases, the egg and sperm may have a normal number of chromosomes, but an error of cell division (nondisjunction) occurs when the fertilized egg is growing and dividing. If an error occurs during one of the divisions, it can cause some cells to have an abnormal number of chromosomes. In people affected by mosaic trisomy 8, some of the body's cells have the usual two copies of chromosome 8, and other cells have three copies of this chromosome (trisomy). The percentage of cells with trisomy 8 and which parts of the body are affected vary from person to person. This leads to variability in the range and severity of symptoms.

• Characteristic facial features such as elongation of the skull (scaphocephaly), prominent forehead, widely-spaced eyes, deeply set eyes, broad upturned nose, and micrognathia
• Brain malformations such as agenesis of the corpus callosum
• Highly arched or cleft palate
• Shortened neck with extra skin folds
• Long slim body with a narrow chest, shoulders, and pelvis
• Kidney and cardiac abnormalities
• Camptodactyly and stiff joints
• Absent malformed kneecap
• Vertebral malformations such as scoliosis
• Eye abnormalities

Most individuals with mosaic trisomy 8 have moderate intellectual disabilities (IQ between 50 and 75), with some people having a normal intelligence. There is no known connection between the percentage of trisomic cells and the severity of the intellectual disability. Mosaic trisomy 8 also may increase the risk for infections, and seems to increase the risk for certain cancers such as Wilms tumor, myelodysplasia, and acute myeloid leukemia.

Because mosaic trisomy 8 affects many different areas of the body, medical management often involves a team of doctors and other healthcare professionals. Treatment for mosaic trisomy 8 varies depending on the signs and symptoms present in each individual. For example, for individuals with delays in motor skills such as walking, physical therapy or occupational therapy may be helpful. Early intervention and special education may be recommended depending on the degree of intellectual disability. If present, cleft palate, cardiac malformations, and kidney malformations may need to be surgically repaired. Other surgeries and interventions may be needed depending on the nature and severity of the other features and their symptoms.

Mosaic trisomy 8 is a rare condition which affects only 1 in every 25,000 to 50,000 liveborn babies. It affects males more often than females. More than 120 people with this condition have been reported in the medical literature.