PAI-1 expression is regulated by a 4G/5G promoter polymorphism. The 4G allele is associated with greater circadian variation of PAI-1 levels. We hypothesized that the 24 h variation of cardiac risk is more pronounced among persons with the 4G4G genotype than among ones with 4G5G and 5G5G genotypes. We assessed the time of onset of symptoms in 623 consecutive patients with acute myocardial infarction (AMI) enrolled in the MISSION! Study between February 1, 2004, and October 29, 2006. All of the patients were genotyped for the PAI-1 4G/5G polymorphism. We quantified the amplitude of the 24 h variation of AMI with a generalized linear model with Poisson distribution. A morning peak, between 06:00-11:59 h (n = 197; 32% of all cases), in the onset of symptoms of AMI was observed. The group composed of patients with the 4G4G genotype did not have a more pronounced morning peak than the groups composed of other genotypes; the 24 h variation was 38% (95% confidence interval 12-70%) in the group of 4G4G patients and 34% (14-58%) and 56% (20-100%) in the 4G5G and 5G5G groups of patients, respectively. Our findings show that 24 h variation of cardiac risk is not more pronounced among the 4G4G genotype of PAI-1.