To examine the relationship between altered cellular immune status and clustered features of the metabolic syndrome, we measured body mass index (BMI), serum concentrations of high-density lipoprotein-cholesterol, triglycerides, fasting plasma glucose, and blood pressure levels as well as differential leukocyte counts and lymphocyte sub-populations among 439 apparently healthy Japanese men aged 35-60 years. The components of the metabolic syndrome were defined based on the following criteria: BMI >/=25.0 kg/m(2), fasting plasma glucose >/=6.11 mmol/l, systolic blood pressure >/=130 mmHg and/or diastolic blood pressure >/=85 mmHg, high-density lipoprotein (HDL)-cholesterol <1.03 mmol/l, and fasting triglyceride >/=1.69 mmol/l. Counts of total leukocyte, total lymphocyte, CD3 + T cell, CD4 + T cell, and CD4 + CD45RO + T cell significantly correlated with the number of components of the metabolic syndrome (0, 1, 2, and 3+) after adjustment for age and smoking status. These findings were more evident among smokers than among non-smokers. The counts of total leukocytes, total lymphocytes and more specifically memory (CD4 + CD45RO + T) cells were elevated with clustered features of the metabolic syndrome in middle-aged men, which suggest the involvement of altered cellular immune status in the pathogenesis of atherosclerosis.