We aimed at seeking more precise diagnostic information on the sensory nervous system involvement described in patients with amyotrophic lateral sclerosis (ALS). We investigated large myelinated nerve fibers with nerve conduction study and small-nerve fibers with Quantitative Sensory Testing (QST) (assessing thermal-pain perceptive thresholds) and skin biopsy (assessing intraepidermal nerve fiber density) in 24 consecutive patients with ALS, 11 with bulbar-onset and 13 with spinal-onset. In 23 of the 24 patients, regardless of ALS onset, nerve conduction study invariably showed large myelinated fiber sparing. In patients with bulbar-onset ALS, QST found normal thermal-pain perceptive thresholds and skin biopsy disclosed normal intraepidermal nerve fiber density. Conversely, in patients with spinal-onset, thermal-pain thresholds were abnormal and distal intraepidermal nerve fiber density was reduced. Sensory nervous system involvement in ALS differs according to disease onset. Patients with spinal-onset but not those with bulbar-onset ALS have concomitant distal small-fiber neuropathy. Neurologists should therefore seek this ALS-related non-motor feature to improve its diagnosis and treatment.