In contrast to findings in ALS, cognitive impairments have as yet not been shown in the lower motor neuron variant of motor neuron disease, progressive spinal muscular atrophy (PMA). Our objective was to investigate cognitive function in PMA and compare the cognitive profile with that of ALS. In addition, we assessed visuopatial functions as these tests are underrepresented in earlier neuropsychological investigations in ALS. Tweny-three PMA and 30 ALS patients (vital capacity >70% of predicted value) underwent a neuropsychological assessment adapted to motor impairments: global cognitive and executive functioning, psychomotor speed, memory, language, attention and visuospatial skills. The results were compared with age, education and sex-matched controls and with normative data. Compared to controls PMA patients performed worse on attention/working memory (digit span backward), category fluency and the mini-mental state examination. Compared with normative data, PMA most frequently showed impairment on three measures: letter-number sequencing, immediate and delayed story recall. Seventeen percent of PMA patients showed cognitive impairment, defined as performance below two standard deviations from the mean of normative data on at least three neuropsychological tests. In ALS similar but more extensive cognitive deficits were found. Visuospatial dysfunction was not found in either of these disorders. Seventeen percent of PMA patients have executive and memory impairments. PMA with cognitive impairment adds a formerly unknown phenotype to the existing classification of motor neuron diseases.