Although subacute thyroiditis (de Quervain's thyroiditis) is presumed to be caused by a viral infection, only 2 familial occurrences of subacute thyroiditis have been reported in the literature. Typical and severe subacute thyroiditis was diagnosed in an older sister who was hospitalized for 8 days. During this period her younger sister nursed her. Three weeks after, the same clinical picture occurred in the younger sister. We postulate that subacute thyroiditis might occur by transmission of possible viral infection in genetically predisposed individuals.