Aim: To investigate whether there is cognitive impairment in patients with motor neuron disease, and analyze the influence of the onset age, disease course, educational level, score of Norris bulbar scale, score of Norris limb scale and the total score of Norris scale on the cognitive function in patients with motor neuron disease. Methods: Thir ty-two patients with motor neuron disease (patient group), who were treated in the Department of neurology, Xuanwu Hospital, Capital University of Medical Sciences between March and December 2003, were involved, their diagnosis all accorded with the El Escorial diagnostic standard revised, by World Neurology Union in 1998, and any other primary central nerve system disease that can cause cognitive dysfunction has been excluded by cranial CT or MRI check. Sixty healthy volunteers (normal control group), who had no neuropsychosis history, and nervous system examination was normal, were matched in age, sex and educational level with the subjects in the patient group. The cognitive functions of the subjects in both groups were evaluated with extended scale for dementia (ESD), including 8 subtests; According to the extent of education from illiterate to university degree holder, the standard scores were 154, 192, 208 and 217 respectively to distinguish different education levels. Hamilton depression scale (HAMD) was used to assess whether the subjects had depression or not, there were 24 items, and 5 grades (0-4 points) were used as the rating system. In addition, the revised Norris scale was applied to measure the bulbar function and limbs function. The range of rating ranged from 0 (the maximum harm) to 99 (healthy). The scores of ESD and HAMD were mainly compared between the patient group and normal control group. The related factors for the cognitive function of patients with motor neuron disease were observed with multiple linear correlation and regression analysis, the measurement data are expressed as Mean±SD, and the enumeration data as rate; The mean of measurement data between the two groups was compared with the t test, while the rate was compared with the chi-square test. Results: All the subjects were involved in the results of analysis. 1 Comparison of mean HAMD and ESD scores: The results of HAMD score showed that there was no significant difference in the incidence rate of depression between the patient group and normal control group (χ 2 =1.458, df =1; P=0.594 4). The ESD total scores, and scores of learning, calculating, attention, speech ability, abstract thinking and memory in the patient group were obviously lower than those in the normal control group (P < 0.05). 2 Related factors of cognitive function in patients with motor neuron disease: The score of bulbar function in the patients with motor neuron disease had positive correlation with the total score of ESD, and the scores of abstract thinking, memory and speech subtests in ESD (P < 0.05). Their educational level was positively correlated with the total score of ESD, and the scores of memory and learning subtests (P < 0.05). The limbs function scores, total score of revised Norris scale, onset age and course of disease had no obvious correlation with the scores of ESD in patients with motor neuron disease (P> 0.05). Conclusion: Cognition dysfunction exists in most of the patients with m otor neuron disease, the characters of the cognitive impairment in the patients are cortical and subcortical and affected by the impaired severity of bulbal function and educational level. The severity of cognitive impairment in patients with motor neuron disease is not parallel to the severity of motor system impairment, it is indicated that neuronal degeneration extends over the motor system in motor neuron disease.