In the past, the main problem of transscleral cyclophotocoagulation was related to the energy dosage of the individual effect. Since the target tissue is not directly observable, no information about the coagulation process was available to the surgeon. High inter- and intraindividual variations of the tissue properties often led to under- or overdosage. Underdosage is therapeutically useless, while overdosage causes the so-called "popeffects" which may induce severe damage to the eye.A small fraction of the impinging laser radiation passes the ciliary body after multiple scattering, is reflected from the fundus, leaves the eye through the optical media and can be recorded by a photodetector outside the eye. The time dependence of this detector signal directly monitors the change in transmission of the coagulated tissue, because all other parameters influencing the signal are constant in time. Using this information, the surgeon or a computer can interrupt the laser process. CLINICAL EVALUATION: In the eyes treated since September 1996, a mean reduction of intraocular pressure was achieved comparable to the values for uncontrolled cyclophotocoagulation reported in the literature. So far, no case of severe complications has been observed.The accuracy and safety of transscleral cyclophotocoagulation are improved by this real-time control procedure. Reduction of risk may allow broader application of the method.