To study the effect of prosthetic replacement in treatment of femoral neck fractures on the hemiplegia side in the elderly. From May 1990 to May 2000, 189 elderly patients with femoral neck fractures were treated with prosthetic replacement in my hospital. Twenty-nine hemiplegia patients, who suffered from stroke previously, had Garden type III and type IV femoral neck fractures on the hemiplegia side. Thirty non-hemiplegia patients were chosen randomly. The two groups were followed-up for 27-98 months (average: 59 months). The age, hospitalization days, operating time, blood loss, blood transfusion, complications during perioperative period and long-term complications were compared between the two groups and the results of femoral head replacement and total hip replacement in the hemiplegia group were also compared. All the patients of the two groups survived the perioperative period. No significant difference was found in the age, hospitalization days, operation time, blood loss and blood transfusion and long-term complications between the two groups (P>0.05). However there was significant difference in complications during perioperative period between the two groups (P<0.05). Five patients died in the hemiplegia group with the mortality of 17.2% and two died in the non-hemiplegia group with the mortality of 6.7% 11 months to 5 years after operation. There was significant difference in long-term complications between the femoral head replacement and the total hip replacement in the hemiplegia group (P<0.05). The result of the total hip replacement was better than that of the femoral head replacement. Prosthetic replacement is a reliable method in treatment of Garden type III and type IV femoral neck fractures on the hemiplegia side in the elderly, and patients are safe during perioperative period. More complications during perioperative period occur in the hemiplegia group, and long-term complications are insignificantly different between the two groups. The mortality rate is higher in the hemiplegia group than in the non-hemiplegia group within 5 years after operation. Since the result of the total hip replacement is better than that of the femoral head replacement, total hip replacement should be chosen firstly to treat Garden type III and type IV femoral neck fractures on the hemiplegia side in the elderly if the muscular strength of the hip is beyond IV degree.