We describe two unrelated children with Kniest dysplasia, a severe autosomal dominant form of chondrodysplastic dwarfism associated with cleft palate, progressive arthropathy, myopia and retinal detachment. In the first patient the disorder was caused by a 28 base pair exon 12/intron 12 deletion in the gene coding for type II collagen. Her mother had mild abnormalities of the vertebral bodies and long bones compatible with abnormalities seen in Stickler arthro-ophthalmopathy. The second child had a transition of AG to GG at the 3' splice site of intron 20 of the COL2A1 gene. Her father had premature polvarthrosis interpreted as a sequela of mild spondyloepiphyseal dysplasia. Molecular studies revealed that the mother of the first and the father of the second child each had somatic mosaicism of the same mutation as their children. Heterozygous mutations of the gene coding for type II collagen can cause Kniest dysplasia, and somatic mosaicism for the same mutations can result in the Stickler phenotype or in mild spondyloepiphyseal dysplasia leading to premature polyarthrosis.