My son is 16, stays over 100%, last visit was 127%! The RT joked and said he almost broke the machine. I agree with Bill, what mutation you have is going to play into it, and there's really nothing you can do about that. I think even with the same mutation, because people are different, each person will react differently and have different levels even if everything else is the same (diet, exercise, meds, type of treatments, etc.). Outside of that, I'm going to say his amount of exercise (he's been playing a lot of airsoft and Ultimate Frisbee- tons of running and jumping) is important, and I'm going to throw in diet, also. The doc collaborated- she said they don't know why, but that weight can really affect lung function. I made a goal to fatten him up before this past visit, and it did really made a difference. MAGIC MILK: add half & half to his milk thermos at lunch, as large of a ratio as he can stand. I put tons of carbs in his lunch, and extra salt so he's thirsty for the milk. I make sure to send him hot food (I bought microwavable hot packs online); hot pasta dishes, lasagna, hot pockets, pizza, hot wings, burritos with rice, etc. because he tends to eat more when he has a hot lunch than a cold one. We were lucky and drew a pretty good card for drawing within the CF suit; we won't overlook it and we are grateful.