We present the case of a 72-year-old woman with slowly progressive spastic paraplegia and painful muscle spasms of the lower limbs. Spastic paraplegia began in the left lower extremity and extended to the right lower extremity 4 months later. We considered the diagnosis of amyotrophic lateral sclerosis (ALS) because of the left-dominant spastic paraplegia of bilateral lower limbs and due to the presence of fasciculation, hyperreflexias, and pathological reflexes. However, cerebrospinal fluid (CSF) examination revealed that cell count and protein values were increased. The patient also had an increased titer of anti-HTLV-1 antibodies in serum and CSF and was diagnosed with HTLV-1 associated myelopathy (HAM). She was treated with steroids, and her symptoms improved. Distinguishing HAM from ALS may be difficult because HAM may present with unilateral spastic paralysis and may be accompanied by fasciculation. Careful and accurate evaluation is necessitated to differentiate between these conditions for a conclusive diagnosis. (Received 1 March, 2021; Accepted 26 April, 2021; Published 1 September, 2021).