To evaluate the efficacy and safety of parenteral nutrition (PN) with lower or higher energy for treating systemic inflammatory response syndrome (SIRS) in the aged, thirty old SIRS critically ill patients were involved in a randomized control clinical trial. Fifteen cases were treated by PN with lower energy (105 KJ.kg-1.d-1), and another fifteen cases with higher energy (146 KJ.kg-1.d-1) for 6 days. The data about metabolisms of protein, glucose and fat, blood gas analysis, acute physiology and chronic health evaluation (APACHE II) were analysed. The results showed that: 1. The effects of 105 KJ.kg-1.d-1 PN in increasing organism protein and maintaining nitrogen balance were the same as those of 146 KJ.kg-1.d-1 PN. 2. The levels of fast blood sugar and triglyceride maintained unchanged in 105 KJ.kg-1.d-1 PN group, while they increased significantly in 146 KJ.kg-1.d-1 PN group(P < 0.05). 3. PaO2 was raised greatly in 105 KJ.kg-1.d-1 PN group, while not in 146 KJ.kg-1.d-1 PN group. 4. 105 KJ.kg-1.d-1 PN and 146 KJ.kg-1.d-1 PN decreased APACHE II(P < 0.05), but the efficiency of 105 KJ.kg-1.d-1 PN was better than that of 146 KJ.kg-1.d-1 PN (P < 0.05). We conclude that PN with 105 KJ.kg-1.d-1 energy intake is more effective and safer for treating SIRS than that with 146 KJ.kg-1.d-1 in the aged.