A previous electroencephalographic (EEG) study has shown that obese subjects are characterized by reduced attentional frontal responses to food images, thus raising the hypothesis of attentional deficits associated with abnormal body weight (Babiloni et al., 2009). In this line, here we tested the hypothesis of reduced attentional cortical responses in underweight subjects. Electroencephalographic data were recorded in 16 normal-weight and 16 underweight subjects during an "oddball" paradigm. The subjects were given frequent (70%) and rare (30%) stimuli depicting faces (FACE), food (FOOD), and landscapes (CONTROL), and clicked the mouse after the rare stimuli. These stimuli depicted the same frequent stimuli graphically dilated by 25% along the horizontal axis. Cortical attentional responses were probed by the difference between positive event-related potentials peaking around 400–500 ms post-stimulus for the rare minus frequent stimuli (P300). Low resolution electromagnetic source tomography (LORETA) estimated P300 sources. The results showed that in the FACE condition, the amplitude of prefrontal (Brodmann area 10 and BA 11) and tempo-parietal (BA 19, BA 20, BA 21, BA 22, BA 36, BA 37, BA 39, BA 40) sources was lower in the underweight than normal-weight subjects. These results suggest that anterior-posterior cortical attentional processes to face images declined in underweight subjects.