Most serious complication is fat embolism. Small amount of fat is injected into damaged blood vessels during fat transfer; it travels in the blood stream to patient lungs, heart, brain, kidney and creates significant problem including death. Lethal outcome was observed in 1 out of 3000-4000 cases of large fat transfer. In all recorded cases where autopsy was done, it was found that fat was injected into muscle or between muscles where mayor blood vessels are located. For that reason most board certified plastic surgeons will now inject fat grafts only into space between muscles and skin of the buttocks. This space is smaller by means of volume, it has relatively limited expansion but seems to be much safer. Because of limited space for fat grafts patient may not have as significant augmentation as it would be possible using all layers of buttocks muscles, between muscles and subcutaneous tissue (space between muscles and skin). Same patients who desire large augmentation may not like these limits but it is in the interest of their own safety. Tromboembolism (legs or lungs) is most frequently seen as a complication of tummy tuck but can be seen in 3-4% of patients undergoing liposuction/fat transfer in particular those who have multiple risks like obesity, birth control, smoking, diabetes, multiple pregnancies, multiple surgeries, history of tromboembolism or family trait. We routinely place these patients on postoperative anticoagulant therapy. However, this condition can happen even 2-3 months after surgery! Hope this helps.