A 16-year-old thoroughbred stallion developed sudden swelling of the left t
esticle. The stallion had previously been regarded as a unilateral cryptorc
hid. Ultrasound examination of the left testicle revealed a diffusely heter
ogeneous parenchyma. The testicle was diffusely hypoechoic with ill defined
regions of hyperechogenicity giving the appearance of hypoechoic nodules t
hroughout the testicular parenchyma. No normal testicular tissue was identi
fiable. An echogenic band, representing a pseudocapsule could be seen surro
unding the testicle. Histopathologic diagnosis a seminoma.