Our objective was to evaluate attentional processing with respect to the clinical-onset subtype in amyotrophic lateral sclerosis (ALS) using event-related potentials (ERPs). Thirty-three non-demented ALS patients (22 spinal onset, 11 bulbar onset) and 32 age- and gender-matched controls underwent a psychophysiological evaluation. Mismatch Negativity (MMN), P300 components and Contingent Negative Variation (CNV) were obtained. Latencies and amplitudes of the MMN, P3a and P3b waves and CNV amplitude were then evaluated. Clinical parameters were correlated with ERP data. No differences emerged between ALS patients and controls with regard to the MMN and P3b components. N1-P3a inter-peak latency (Fz, p = 0.003; Cz, p = 0.001; Pz, p = 0.002) was longer in ALS-b than in ALS-s. Total CNV area (Cz, p = 0.01) and W1-CNV area were significantly reduced (Cz, p = 0.05; Pz, p = 0.03) in ALS-b with respect to the one of the controls, while no differences were found between ALS-s patients and controls. In conclusion, automatic pre-attentive processing of stimuli seems to be preserved in ALS. However, a significant delay in the time-course of selective attentive processing and a difficulty in initiating and sustaining attention may be present in ALS-b, which points to a possible dysfunction in the frontal neural network that responds to novelty and to abnormal integration of associative functions. This attentional impairment should be taken in account while developing alternative communicative strategies in ALS patients.