Abstract.The histopathological effect of photodynamic therapy (PDT) was investigated in an intraocular retinoblastoma‐like tumour in vivo. Eighty‐two tumours were studied by light microscopy and 8 by electron microscopy. Damage of the vascular endothelium with dilation of the organelles was evident 1 h after treatment, followed by leakage of the blood vessels, tissue hemorrhages, and vascular collapse. Histopathological examination showed an overall pattern of shrinkage of the cytoplasm and pycnosis of the nuclei in most of the tumour cells 3–5 days after treatment. The tumour recurrence often developed from the periphery of the tumours and in a few cases from small islets of viable tissue. Use of high doses of Photofrin II or light energy was associated with damage in the light irradiated area both to the conjunctiva or cornea in the form of leucocyte infiltration or ulcers, and to the retina, which often developed edema and appeared severely disorganized, with damage of the photorece