Thanks for putting up your query at HealthCareMagic. I am sorry to hear that your 1 year old son has a swollen testicle. However, there are a few more things that I would have liked to know. Assuming that the swelling is actually affecting the testicle/testis and not any other structure in the scrotal sac, the first thing I would like to know is whether it was present since birth or whether it has developed recently. The second thing is whether it is associated with any redness and pain when touched. The third thing that I would like to ask is whether there is fever or any other symptoms present alongwith. Now let me come to the possibilities one by one. If it is present since birth (congenital), it could be some sort of tumor or even a simple hydrocele. That however has less possibilities of being associated with pain. If however, redness or pain is present, a condition called torsion of testis may be present wherein the blood supply to a testis is disrupted. The pain and redness could also arise when there is inflammation of the duct of the testis known as the epididymis with or without affecting the testis, a condition called epididymo-orchitis. This could also be caused due to an urinary tract infection or a viral infection like mumps. This might be associated with fever. In some conditions an abscess or a collection of pus might also develop. I think that I have been able to give you an overall idea but if you get back with more details, I could perhaps be more precise. In light of the things I have told you, you might also like a briefing from your doctor on the possibilities he is considering.