Hi Maryann, Congrats on the no change MRI!! I think that everyone's doctor has a different "protocol" ...probably dependent on the doctor's level of aversion to risk. And then it depends, of course, on the nature of someone's MS, how long they have had MS, what their MRI looks like as well. I get the sense that there are still a lot of unknowns with this drug so doctors make their own call. I have decided that I will be going off it soon. I've had 22 infusions now. I LOVE Tysabri but since my MS is mild the doctor thinks the risk is probably not worth it at this moment. I do have to agree. I went on Tysabri because I couldn't tolerate interferons or copaxone and there were no PML cases at that point. Now there are a lot. However, if I test negative for the antibodies (once my clinic gets that set up--hopefully soon) I will definitely stay on the drug. Natalie