We present the case of a boy with Costello syndrome who developed osteofibrous 
dysplasia during a phase of growth hormone therapy. The lesion and the 
accompanying pain disappeared after discontinuation of the therapy. Growth 
hormone is a known mitogen for some neoplasms and osteofibrous dysplasia has 
been reported to become aggressive. Thus, although osteofibrous dysplasia in 
Costello syndrome has not been reported before, growth hormone therapy should be 
used under close supervision in children with this syndrome.