Lynn- You've picked the right tight for IT support. Since you are tall and 138lbs, you are really between two sizes. Your height calls for a Large, but if your weight points to Medium. If your height is mostly in your legs, then I would say you need a size large. Especially if you have strong, athletic legs. if your height is mostly in your upper body and your legs are skinny, I would try the medium. You may have a small gap between the bottom of the tight and your ankle, but that's ok. The tights should feel comfortable and fit like second skin. Not too tight and definitely not too loose. You know the tight is too loose on your legs if you see major wrinkles in the fabric and it's bunching up. I hope you enjoy the tights.