Two siblings, a 38-year-old woman and a 33-year-old man, developed eosinophilic fasciitis within a period of 6 months. They were found to have identical HLA-A, B, DR, and DQ antigens, raising the possibility of a genetic influence in the development of this disease. No common environmental factors close to the time of onset were identified; however, the possibility of a common, remote environmental factor cannot be discounted.