I'm finding different answers on the internet, but first I need to clarify what I mean by the term "progressive." I think we have all experienced our symptoms getting worse. The question I'm asking is whether it keeps getting worse your whole life (without effective treatment, of course), or whether the symptoms reach a maximum severity after a certain amount of time.

"Maximum weakness occurs during the first year in two-thirds of patients. In the era before corticosteroids were used for treatment, approximately one-third of patients improved spontaneously, one-third became worse, and one-third died of the disease. Spontaneous improvement frequently occurred early in the course. Symptoms fluctuated over a relatively short period of time and then became progressively severe for several years (active stage). The active stage is followed by an inactive state in which fluctuations in strength still occurred but are attributable to fatigue, intercurrent illness, or other identifiable factors. After 15 to 20 years, weakness often becomes fixed and the most severely involved muscles are frequently atrophic (burnt-out stage)." source: http://www.myasthenia.org/HealthProf...rviewofMG.aspx

This is a different picture from ALS, for example, in which the symptoms progress until the patient dies.

Here's another one:

"Symptoms of myasthenia gravis usually progress to maximum severity within 3 years." source: http://www.mda.org.au/Disorders/NMJ/MG.asp

Here's another:

"In most patients, the severity of disease approaches and may attain a maximum during the first 1 to 3 years." source: Myasthenia Gravis and Myasthenic Disorders By Andrew Engel (I found this here: http://books.google.com/books?id=aKE...ity%22&f=false)]

If you look up the last quote, you'll see statistics about the size of the study, etc. I conclude that there is at least some evidence that unlike some other chronic neuromuscular diseases, myasthenia gravis often stabilizes after a certain amount of time, instead of continually progressing in severity.

Abby