Thickness of the extraocular rectus muscles can be displayed and measured by non-standardized A-scan echography. To establish a norm for the thickness of the rectus muscles, 100 eyes of 50 subjects were examined, with the result that the four recti ranged from 3 mm to 5 mm in thickness. The methodology and the normal range derived may be helpful in the diagnosis and differentiation of diseases affecting the extraocular muscles, such as Graves' disease and myositis.