The ultrasonographic and histopathological findings in 12 normal and 28 injured superficial digital flexor tendons, with lesions ranging in duration from 2 days to 15 months, were compared. A consistent relationship between the ultrasonographic and histological findings was demonstrated. The echogenicity of lesions, the distinctness of their delineation from the surrounding tissue, and the presence and arrangement of the linear echoes were useful features by which to assess the ultrasonograms. Acute lesions were anechoic, a complex mixture of anechoic and hypoechoic areas, or diffusely hypoechoic. These appearances represented haemorrhage, fibrolysis and early granulation tissue. Fibroplasia and granulation tissue produced well to moderately well defined hypoechoic lesions. Chronic fibrosis was characterised by heterogeneously echogenic areas which were poorly defined from the surrounding tissue and had irregularly-arranged linear echoes on longitudinal images. Intratendinous scar formation resulted in multiple hyperechoic foci. Extensive peritendinous lesions were readily apparent on ultrasonograms, but intertendinous adhesions were more difficult to assess, and produced ill-definition of the borders between the superficial and deep digital flexor tendons.