This is a great question! There are many different underlying causes of ptosis, and depending on the reason behind it, the answer to "will it progress if left untreated" will change. In general though, most causes off ptosis will slowly progress over time. Forehead compensation with ptosis is referred to as pseudoptosis, and again this compensation for one sided ptosis will vary depending on the amount of asymmetry present. This pseudoptosis "correction" may or may not fully compensate for the ptosis. This compensation works by causing contraction of the forehead muscle (frontalis) which raises the brows and to some extent the eyelids. This is the same muscle that causes the horizontal forehead wrinkles to worsen and deepen, which is one of the main reason patients receive facial botox - to fix these wrinkles. Bottom line is, the ptosis will most likely slowly progress and may one day require correction if it starts causing an upper visual field defect. I recommend you consult with a board eligible or board certified plastic surgeon for a full evaluation and to discuss the specifics of your case and the options moving forward. Best of luck!