The hereditary spastic paraparesis (HSP) is a group of rare heterogeneous neurodegenerative disorders that share the principal clinical feature of progressive lower limb spastic paralysis. They are classified into pure (pHSP) and complicated (cHSP) forms depending on whether the paraparesis exists alone or in combination with other major clinical features. Cognitive impairment is a frequent feature in cHSP variants and includes the occurrence of mental impairment, dementia, isolated cognitive abnormalities or premature cognitive decline. The aim of the study was to find out if and to which extent a neuropsychological impairment occurs within the different forms of HSP. We evaluated 20 pHSP patients and 11 cHSP patients in comparison to an age-, sex-, and education-matched control cohort. A comprehensive test battery covering a broad range of cognitive functions as well as a screening for dementia was done. Tests were as follows: Digit Span, Block-Span, California Verbal Learning Test, Symbol Digit Modalities Test, Coloured Progressive Matrices, Controlled Oral Word Association Test, and Mini-Mental Status Test. Furthermore, we calculated the brain parenchymal fraction (BPF) from 3-D MRI data in all patients. BPF can specify the presence of global brain atrophy or (in an adapted processing step) changes in the white or the grey matter subfractions. The neuropsychological and the BPF data were correlated. In the pHSP group, significant impairments were observed in memory testing only. Correlation to the MR data showed the screening for attention and dementia. The cHSP patients were characterized by a significantly reduced test performance in comparison to its control group, irrespective of the domain that had been assessed. In addition, nearly all different neuropsychological test modalities showed significant correlations to the BPF and the grey and white matter.