Neutering is often associated with obesity in companion animals. However, the maintenance energy requirement (MER) for these animals has not been clearly defined. The present study investigated the MER for spayed cats whose body weights (BW) began to increase shortly after ovariohysterectomy. A total of twenty-two shorthair adult female cats were fed complete and balanced diets in amounts to maintain their BW and body condition score (BCS) before the present study. All cats were spayed and the diet was fed for 11 weeks using the same MER as previously. During these weeks, all cats gained weight. Beginning with week 12, a weight-loss regimen was initiated until each cat achieved a BCS of 5 out of 9. After each cat obtained a BCS of 5, an appropriate amount of diet was fed to maintain its BW for at least 4 weeks to determine a modified MER. Daily food consumption, weekly BW and BCS were monitored. Blood was collected before and after weight loss for plasma biochemistry profiles. BW and BCS increased by 16 % and one point (P < 0·01), respectively, during the first 11 weeks after surgery, although food consumption was constant both pre- and post-surgery. The mean MER after obtaining a BCS of 5 was 313·6 (sem 23·6) kJ/BW0·67, which is 25 % lower than the current National Research Council recommendation and lower than the cat's requirement before surgery (P < 0·05). In conclusion, spaying significantly increased BW when using MER values for intact cats. Thus, 313·6 × ideal BW0·67 kJ is proposed for the MER of spayed adult cats.