We report three male patients of younger age group presented with acute vein thrombosis of different sites, right lower limb, cortical venous sinuses thrombosis with cerebral vascular accident and third case is mesenteric vein thrombosis. All patients were Vegetarian, had low level of cobalamin with marked hyper homocysteinemia with normal serum and red cell folic acid. The low Cobalamin level was not suspected secondary to pernicious anemia, based on the fact that there was no evidence of atrophic gastritis and an absence of antiparietal cell antibodies. There were no evident of immobilization, recent surgery, malignancy, antiphospholipid antibody, myeloproliferative disorder, and hormone replacement therapy. No deficiencies in protein C, protein S, or antithrombin III, normal factor V Leiden, no prothrombin gene mutation 20210A and no clone for paroxysmal nocturnal hemoglobin-urea were detected, no cause was found for the thrombosis apart from their secondary hyperhomocysteinemia.