It has been suggested that bipolar disorder is associated with altered immune function. Monocyte chemoattractant protein-1 (MCP-1) is a chemokine that influences both neural and immune functions. We thus hypothesized that MCP-1 may be related to the development or pathophysiology of bipolar disorder. In this case-control study, we investigated the association between the A-2518G single nucleotide polymorphism (SNP) of the MCP-1 promoter and bipolar disorder. Patients with bipolar disorder (n=183; bipolar I=145, bipolar II=38) and healthy controls (350) were recruited for the study. No significant allelic or genotypic association was detected between the A-2518G polymorphism and any sample of bipolar disorder patients. When we pooled the healthy controls and the cases of bipolar I disorder from previous Korean studies and this study, we again found no significant association. No significant difference in either allele frequency or genotype distribution was observed between bipolar I and bipolar II disorders. There was no difference in the age at onset of bipolar disorder among the three genotype groups. Our data suggest that the A-2518G polymorphism of MCP-1 is not a major susceptibility factor for bipolar disorder in the Korean population. However, the physiological role of MCP-1 is highly suggestive of its being associated with bipolar disorder, and further analyses of other SNPs of MCP-1 remain to be performed.