Accumulating evidence suggests that Rho-associated kinase (ROCK) may be important in the pathogenesis of atherosclerosis and coronary vasospasm. In the present study, we investigated whether ROCK activity is increased in acute coronary syndrome (ACS) patients. Twenty-one patients with ACS (12 males, mean age 58.0 +/- 8.0 years) and 20 control subjects (10 males, mean age 55.0 +/- 6.0 years) were enrolled. Blood samples were obtained and demographics were recorded. Peripheral leukocyte ROCK activity was determined by the ratio of phospho-myosin-binding subunit (P-MBS) on myosin light-chain phosphatase to total MBS. Compared with the control subjects, ROCK activity was significantly increased in ACS patients (0.69 +/- 0.07 vs. 0.45 +/- 0.04, P<0.001). There was no apparent correlation between the lipid levels (total cholesterol and low-density lipoprotein) and ROCK activity (r=0.17, P>0.05; r=0.08, P>0.05; respectively). However, ROCK activity correlated with mean arterial pressure (r=0.58; P<0.01). ROCK activity is increased in ACS patients indicating that this may be a novel serological marker of ACS.