My son is 4 years old, 38 lbs, 41" tall, I feel he is thin to avg so I ordered him a size 2/3T, they fit him perfect! They do shrink a little from out of the package, but they are his favorite. My son chooses these over all the character undies, which NEVER fit him right:(

I would give these a 5-star, but I do feel they should be a tad longer (in length). I feel these run true to size. If your son is slim to average, I suggest a size smaller, if you want them to fit JUST right. I am back ordering him another pack of 2/3T and some 4T's to grow into, as it took me 2 years to find just the right underwear for him! Who knew underwear could be so difficult to fit:)