- Increased homocysteine plasma levels were reported in patients with schizophrenia and Levine et al. (2002) suggested that such increase characterizes mainly males. In the following study we examined whether such increased levels also characterize male siblings of schizophrenia patients. Forty-four pairs of schizophrenia patients and their corresponding healthy male siblings were recruited and sampled for homocysteine. We also had age-matched controls for each of the sibling. The median homocysteine plasma level for patients was 13.0 µMol/L and 11.7 µMol/L for their male siblings compared with a median of 10.9 µMol/L for the siblings' controls. There was no significant difference between homocysteine plasma level in patients and their siblings. Significant difference was found for homocysteine plasma level between the siblings' group and their matched controls. A partial correlation of Ln plasma homocysteine level between patients and their siblings was found to be close to a zero correlation of −0.089, p=0.57 for the whole study group and −0.15, p=0.38 in the male–male patient-sibling pairs. Our results show that elevated homocysteine plasma level may characterize schizophrenia patients' male siblings, a finding that seems to agree with previous studies suggesting elevated homocysteine level as a risk factor for developing schizophrenia.