Two cases of an unusual association - fibrodysplasia ossificans progre
ssiva and synovial chondromatosis - in non-related children are presen
ted. This association does not seem coincidental and raises several qu
estions about the pathogenesis. A genetic hypothesis related to G prot
eins is proposed. This is supported by the fact that such abnormalitie
s have been demonstrated in pseudohypoparathyroidism and fibrous dyspl
asia; these diseases can also be associated with fibrodysplasia ossifi
cans progressiva.