An increased plasma level of homocysteine is called hyperhomocysteinaemia and may be caused by several factors like gene mutations, vitamin deficiencies, drug interactions, renal failure etc. The non-essential sulphur containing amino acid O homocysteine holds a crucial position in remethylation and transsulphuration path- ways. Its interconversion between other metabolites is catalyzed by enzymes requiring folate, vitamin B12 or vitamin B6 for their correct function. Using the L-methionine loading test (100 mg/kg b.w.) before and after supplementation with the above mentioned vitamins it is possible to differentiate among hyperhomocysteinaemia s and to introduce adequate treatment for reduction of plasma homocysteine. Among 6 541 patients mostly suffering from arterial and coronary occlusions 289 loading tests with L-methio- nine were made so far. After supplementation of patients with vitamins, 70% of them were folate responsive, 9% folate and pyridoxine responsive, 6% vitamin B12 responsive and 15% were resistent to any vitamin supplemen- tation. Typical curves of fasting homocysteine and 6 hrs after the L-methionine load are shown in selected typical patients with renal failure, in patients with folate, pyridoxine, vitamin B 12 deficiency and in epileptic patient on carbamazepine treatment.

        Key words: total homocysteine, hyperhomocysteinaemia, differential diagnosis, m ild hyperhomocysteinaemia, L-methionine-loading test.