The brown light reflection in the anterior chamber of the eye lens is thought to be a “mercury lensitis” caused by mercury deposition, which can persist after the symptoms of toxicity have disappeared or after exposure to mercury has been removed, and is another marker of mercury absorption. How is the brown light reflex in the anterior chamber of the eye diagnosed? The diagnosis of chronic mercury poisoning should emphasize the history of exposure, the main clinical manifestations such as psycho-neurological symptoms, stomatitis and tremor, and the need to exclude similar clinical manifestations caused by other etiologies. Increased values of urine mercury and blood mercury can be useful for diagnosis. The mercury repellent test can be performed with sodium dimercaptopropionate 0.25g, injected intramuscularly; or disodium dimercaptobutyrate 0.5g, injected intravenously; if the urinary mercury excretion is significantly increased, it can be used as an important auxiliary diagnostic basis. It is often caused by occupational inhalation of mercury vapor, and in a few patients it can also be caused by the application of mercury preparations. Psycho-neurological symptoms may start with dizziness, headache, insomnia, and dreaminess, followed by emotional excitement or depression, anxiety and timidity, and manifestations of phytodysfunction such as flushing, excessive sweating, and skin scratching signs. Muscle tremors are first seen in the fingers, eyelids and tongue, and later in the arms, lower limbs and head, or even the whole body; they are more pronounced when being noticed and excited. Oral symptoms mainly include mucosal congestion, ulcers, swollen and bleeding gums, and loose and lost teeth. In cases of poor oral hygiene, blue-black lines of mercury sulfide particles arranged in rows can be seen in the gums, which is a marker of mercury absorption. In the kidneys, there is initially subclinical renal tubular impairment with low-molecular proteinuria, but also nephritis and nephrotic syndrome. Renal damage can be expected to recover after removal from mercury exposure. Patients with chronic toxicity may also have weight loss, hypogonadism, menstrual disorders or miscarriage in women, as well as hyperthyroidism and peripheral neuropathy. The brown light reflex in the anterior chamber of the ocular lens is thought to be caused by mercury deposition “mercury lensitis” and this brown light reflex can persist after the symptoms of poisoning disappear or after removal from mercury exposure, which is another marker of a mercury absorption. The diagnosis of chronic mercury poisoning should emphasize the history of exposure, the main clinical manifestations such as psycho-neurological symptoms, stomatitis and tremors, and the need to exclude similar clinical manifestations caused by other etiologies. Elevated values of urine mercury and blood mercury can be useful for diagnosis. The mercury repellent test can be performed with sodium dimercaptopropionate 0.25g, injected intramuscularly, or disodium dimercaptobutyrate 0.5g, injected intravenously; if the urinary mercury excretion is significantly increased, it can be used as an important auxiliary diagnostic basis.