We investigated the inhibitory effect of caffeic acid phenethyl ester (CAPE) on the differentiation of 3T3-L1 mouse fibroblasts to adipocytes. 3T3-L1 cells were differentiated for adipocytes given high glucose Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 1 microM dexamethasone (DEX), 500 microM isobutylmethylxanthine (IBMX), and 5 microg/ml insulin for 7 days. After differentiation, cells were stained with Oil-Red-O to detect oil droplets in adipocytes. Additionally, the cells were lysed and measured for triglyceride contents. Total RNA was isolated from differentiated cells on day 0, 4 and 7. Then, RNA was analyzed using reverse transcription (RT)-polymerase chain reaction (PCR). CAPE dose-dependently suppressed oil droplet accumulation and reduced the droplet size. These findings showed that CAPE at concentrations of 25 to 50 microM could significantly inhibit triglyceride deposition (p<0.05). Treatment of 3T3-L1 with CAPE reduced the mRNA levels of peroxisome proliferator-activated receptor (PPAR) gamma and CCAAT/enhancer-binding protein (C/EBPalpha). Fatty acid synthase (Fas) and adipocyte-specific fatty acid binding protein (aP2) are known to be associated with lipid metabolism in adipocytes, and both Fas mRNA and aP2 mRNA were significantly suppressed by CAPE treatment. These findings suggested that CAPE suppresses 3T3-L1 differentiation to adipocytes through inhibition of PPARgamma, C/EBPalpha, Fas and aP2 expression.