Hereditary spastic paraparesis (HSP) caused by mutations in the SPAST (SPG4) gene are autosomal-dominant inherited disorders characterized by weakness of lower extremities, spasticity and hyperreflexia. Some cases with cognitive decline have been repored. Herein we present an asymptomatic carrier of a SPAST gene mutation who developed an adult-onset cognitive decline, compatible with Alzheimer's disease with co-pathologies such as argyrophylic grain disease and cerebrovascular pathology. No pathological changes described in HSP patients were present in this case.