Three patients underwent pars plana vitrectomy for vitreous hemorrhage secondary to an intraocular malignant melanoma. In all three cases the tumour was unsuspected and had been missed during pre-operative ultrasound examination. In each case vitreous haemorrhage was caused by invasion and perforation of the sensory retina and tumour growth into the vitreous cavity with bleeding from large vessels near the tumour surface (Knapp-Rønne growth pattern). One 75-year-old patient still has useful vision 3 years after surgery; in two patients the eyes were enucleated. Histology revealed in one case a diffuse spreading of tumour cells covering all intraocular surfaces: the posterior and anterior surface of the iris, the angle structures, the posterior corneal surface, and most of the inner retinal surface--probably due to transformation of growth pattern following vitrectomy.