We have fabricated label-free protein biosensors based on aptamer-modified carbon nanotube field-effect transistors (CNT-FETs) for the detection of immunoglobulin E (IgE). After the covalent immobilization of 5'-amino-modified 45-mer aptamers on the CNT channels, the electrical properties of the CNT-FETs were monitored in real time. The introduction of target IgE at various concentrations caused a sharp decrease in the source-drain current, and a gradual saturation was observed at lower concentrations. The amount of the net source-drain current before and after IgE introduction on the aptamer-modified CNT-FETs increased as a function of IgE concentration. The detection limit for IgE was determined as 250 pM. We have also prepared CNT-FET biosensors using a monoclonal antibody against IgE (IgE-mAb). The electrical properties of the aptamer- and antibody-modified CNT-FETs were compared. The performance of aptamer-modified CNT-FETs provided better results than the ones obtained using IgE-mAb-modified CNT-FETs under similar conditions. Thus, we suggest that the aptamer-modified CNT-FETs are promising candidates for the development of label-free protein biosensors.