We have established a reference standard for the cross-sectional area (CSA) of supraspinatus as measured by diagnostic ultrasound. The influence of hand dominance and of ageing on the CSA was also assessed. We examined 72 subjects aged from 20 to 79 years. Standard values of the CSA were determined with a high measure of interobserver reliability. Although the CSA on the dominant side was significantly larger (p < 0.001) by 0.16 cm2 (95% CI 0.072 to 0.249) than that on the non-dominant side, this difference had no clinical significance. The CSA of supraspinatus decreased significantly with ageing.