A xanthomatous lesion resembling balloon cell melanoma of the iris and ciliary body of a dog.

A slow-growing xanthomatous tumour which diffusely infiltrated the iris and ciliary body of a dog was composed of large, pale, vacuolated cells which contained scattered fine pigment granules. Electron microscopy revealed coalescing vacuoles, lamellar membranous structures, and small groups of melanosomes in the cytoplasm, suggesting melanocytic origin. The lesion resembled balloon cell melanoma of the ciliary body of man. The behaviour of the tumour was benign. This is a clinical and pathological entity which has not been reported previously in the dog.