Although photodynamic therapy has shown great promise for the treatment of a variety of malignant neoplasms, the role of this new therapeutic modality in the clinical management of intraocular tumors remains incompletely understood. This study examines the effects of photodynamic therapy using chloroaluminum sulfonated phthalocyanine on Greene hamster melanoma transplanted into the subchoroidal space in rabbits. Twenty-four hours after intravenous administration of chloroaluminum sulfonated phthalocyanine (5 mg/kg), tumors were irradiated with 675 nm of light at total light doses of 7 to 60 J/cm2. The results show that tumor growth was arrested at total light doses of 22 to 60 J/cm2. At total light doses of 15 to 21 J/cm2, tumor growth was initially arrested. However, regrowth of these tumors was apparent within 7 days. Total light doses of less than 15 J/cm2 showed no response. Complications of photodynamic therapy, such as intraretinal or subretinal hemorrhages and retinal detachment, were seen only in animals who received total light doses in excess of 43 J/cm2.