Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of motor neurons and appearance of skein‐like inclusions. The inclusions are composed of trans‐activation response (TAR) DNA‐binding protein 43 (TDP‐43), a member of the heterogeneous nuclear ribonucleoprotein (hnRNP) family. hnRNPA1 and hnRNPA2/B1 are hnRNPs that interact with the C‐terminus of TDP‐43. Using immunohistochemistry, we investigated the association between TDP‐43 and hnRNPA1 in ALS spinal motor neurons. We examined spinal cords of seven ALS cases and six muscular dystrophy cases (used as controls) for the presence of TDP‐43 and hnRNPA1 protein. In the control cases, hnRNPA1 immunoreactivity in motor neurons was intense in the nucleus and weak in the cytoplasm where it showed a fine granular appearance. In the ALS cases, hnRNPA1 immunoreactivity in motor neurons was reduced in the nuclei of neurons with skein‐like inclusions but was not detected in the skein‐like inclusions. The marked loss of hnRNPA1 in motor neurons with concomitant cytoplasmic aggregation of TDP‐43 may represent a severe disturbance of mRNA processing, suggesting a key role in progressive neuronal death in ALS.