Hyperhomocysteinemia has been described as a risk factor for venous and arterial thromboembolic diseases but may be also involved in neurodegenerative and neuropsychiatric disorders. Considering some arguments for homocysteine (Hcy) toxicity, a systematic investigation tool of hyperhomocysteinemia is needed. Understanding of the complexity of homocysteine metabolism can help to improve etiologic diagnosis of hyperhomocysteinemia, especially in determining the genetic or nutritional origin of the anomaly. We propose here a brief description of different clinical presentations and a strategy for biological investigation of hyperhomocysteinemia.