Plasminogen activator inhibitor-1 (PAI-1) plays an inhibitory role in the fibrinolytic enzyme system and is associated with thrombotic diseases. The gene for PAI-1 has an insertion/deletion polymorphism at the promoter region, the 4G/5G polymorphism, which is related to differences in transcription activity in vitro. Association of the 4G/5G polymorphism with plasma PAI-1 levels, however, has not been uniformly reported. We evaluated the relationship between the 4G/5G polymorphism and plasma PAI-1 antigen levels in 104 Japanese males not taking lipid-lowering drugs and without non-insulin-dependent (type 2) diabetes mellitus or coronary artery disease. The genotype frequency was 37.5%, 50.0%, and 12.5% for 4G/4G, 4G/5G, and 5G/5G genotypes, respectively, which differs from that reported for healthy Caucasian males (P < 0.01), with the 4G allele more frequently found in the Japanese population. No association was found between the 4G/5G polymorphism and plasma PAI-1 antigen levels. Multiple regression analysis revealed a significant (P < 0.01) contribution of triglyceride (TG) levels to variations in plasma PAI-1 antigen levels. The correlation between TG levels and plasma PAI-1 antigen levels was not 4G/5G genotype-specific. These findings suggest that PAI-1 4G/5G polymorphism is not associated with plasma PAI-1 antigen levels among healthy Japanese males and that TG levels correlate to plasma PAI-1 antigen levels in all PAI-1 4G/5G genotypes.