Eightyeightkeys, I was able to put weight on through protein shakes that I made with either frozen yogurt or ice cream (bought the shake mix through a vitamin and health food store) and strength training. It took me some time, but I was able to gain ten pounds in a few months. I was 90 pounds and was able to get back up to 100 pounds (I'm 5'2"). Mind you I'm also old (late twenties) and a mom of two. Ask your doctor for advice as well. I was very sick after my first child was born and lost a ton of weight. Thus my resolve to gain those last ten which I did. After my second child it went back to normal and I didn't have any drastic weight losses. Hope this helps!