Classical homocystinuria is a rare autosomal recessive inherited disorder. It is caused by a deficiency of the enzyme cystathionine-ß-synthase leading to accumulation of homocysteine and methionine. High levels of plasma homocysteine are associated with vascular injury via mechanisms of oxidative damage, vascular smooth muscle proliferation, promotion of platelet activation and aggregation, and disruption of normal procoagulant-anticoagulant balance favouring thrombosis. The current cumulative detection rate of CBS deficiency is 1 in 344,000. Classical homocystinuria involves four major organ systems: eye, skeletal, central nervous system and cardiovascular system. It can also involve liver, skin and hair. The diagnosis is usually confirmed by the presence of elevated levels of total homocysteine and methionine in plasma. Diagnosis at early age should aim to prevent ocular, musculoskeletal and intravascular complications and to ensure normal development. However, late diagnosis should focus at reducing life threatening thromboembolic events. There is no cure for homocystinuria. One third of the patients with CBS deficiency are vitamin B6 responsive, and they do not need other treatments. Patients with BCS not B6 responsive are treated with a low-methionine diet, vitamin B6, vitamin B12, folic acid and betaine. These patients most commonly present for ophthalmic surgeries for ectopia lentis. Fatal episodes of thromboembolic phenomena may occur in these patients following general anaesthesia if not diagnosed before or if there is poor metabolic control. To minimize these risks we suggest good compliance to diet and treatment, avoidance of dehydration and hypoglycemia,  reduction in blood viscosity and platelet aggregation by medication (if poor metabolic control), and the maintenance of good venous return, avoidance of nitrous oxide and the postoperative early ambulation of the patient.