This study was performed to assess mortality and functional outcome after hemi-arthroplasty for displaced intracapsular femoral neck fractures in relation to mental state. Between 1991 and 1995, 202 consecutive patients over 70 years of age were followed for at least two years or until death. Thirty-nine patients were known with senile dementia at the time of admission. The four-month mortality rate was 11.7% for the mentally normal patients and 33.3% for the mentally impaired patients. After one year the mortality rate was 19. 6% for the mentally normal patients and 43.6% for the mentally impaired patients. This difference is statistically significant (p<0. 001). Of the 141 surviving mentally normal patients, who had been mobile before operation, 16 (11.3%) were not mobile four months after operation. Of the 24 surviving mentally impaired patients, who had been mobile before operation, 18 (75.0%) were not mobile four months after operation. This difference is statistically significant (p<0.001). The conclusion of our study is that mental state has a statistically significant effect on mortality and functional outcome after hemi-arthroplasty for displaced intracapsular femoral neck fractures. For demented patients, hemi-arthrplasty is a too major operation and less invasive methods of internal fixation should be considered.