'Triggering receptor expressed on myeloid cells 2' (TREM2) gene is involved in Alzheimer's disease (AD) andTREM2mRNA expression is known to be increased in the peripheral blood cells of AD patients. In this study, we examined the expression levels of TREM2 mRNA in peripheral leukocytes of early and late-onset AD patients. We have also investigated the effect of the presence of APOE epsilon 4 allele onTREM2expression.TREM2mRNA expression was analyzed in 30 early-onset AD (EOAD) patients, 38 late-onset AD (LOAD) patients, and in their age-matched controls by using quantitative real-time polymerase chain reaction.TREM2levels in LOAD patients were higher than EOAD. Also, in elderly controls significantly higherTREM2levels were found compared with young controls. Moreover,APOE epsilon 4 carriers in LOAD patients exhibited significantly higherTREM2expression levels thanAPOE epsilon 4 non-carriers and elderly controls. Also, correlation analysis showed thatTREM2mRNA expression was increased by age. The differential expression ofTREM2mRNA levels between EOAD and LOAD patients might be independent of the AD disease status and results from an age-related increase inTREM2expression. In LOAD patients, increased age and the presence ofAPOE epsilon 4 allele further increaseTREM2expression. Taken together, we can suggest that age is a factor that increasesTREM2expression, andTREM2andAPOE epsilon 4may interact together in the pathogenesis of LOAD.