Purpose of the study: The purpose of the study was evaluation of effectiveness of diode laser transpupillary thermotherapy and determining in what extent it may act as the only method of therapy of choroidal tumors, and when it should be used in combination with other methods. We evaluated also the effectiveness of transpupillary thermotherapy (TTT) in generating tumor necrosis by light (LM) and electron microscopy (EM), as well as additional cell damage in the area directly adherent to the necrotic zone. Patients and method: 57 eyes of 55 patients aged 31 to 82 years (mean – 55 years) and diagnosed with intraocular tumor were evaluated. The follow-up time ranged from 2 to 25 months. All patients were treated with transpupillary thermotherapy (TTT) using diode laser OcuLight SLx, IRIS Medical Instruments, Inc. Five eyes of five patients were enucleated because of inefficient treatment and three globes were evaluated in regard to effectiveness of TTT by light (LM) and electron microscopy (EM). Results: In 41 cases (72%) of 57 eyes tumor regression, in 7 eyes (12%) stabilization and in 9 (16%) progression was stated. As to complications and side effects visual field scotomas in treated area, hemorrhage to the vitreous, PVR, secondary neovascular glaucoma and exudative retinal detachment were found during the treatment and follow-up. On histopathological examination with an aid of LM we found three zones of changes, but EM revealed additional zone D which was invisible on LM. No scleral alterations induced by heat were found. Conclusions: 1. In selected cases transpupillary thermotherapy (TTT) can be effective especially in regard to small tumors. 2. Treatment of larger tumors should be managed rather in combination with other methods (e.g. 106Ru) in order to speed-up the treatment and increase the chances for total tumor regression. 3. Depending on clinical image and obtained therapeutic effects the treatment should be multiply repeated. 4. After TTT the cytotoxic effect gradually decreases proportionally to the distance from the central point of the diode laser spot with additional cell damage in the area adjacent to the necrotic zone. 5. The finding of an additional area of seriously injured cells may explain the prolonged time that is needed for tumor regression after TTT and the radiotherapeutic effect on this area in case of simultaneous plaque radiotherapy which may finally result in a deeper tumor necrosis compared to TTT as sole therapy. 6. All complications and side effects stated in the course of treatment were not dangerous to the eye, however in some cases further treatment may be problematic or even impossible.