My girlfriend just told me she is pregnant. I had a vasectomy 5 years ago?

You can't just do a semen test and determine you didn't father this child. DNA is the ONLY way. Why? Recization of the vas defernes can happen for different periods of time. Meaning the vas may open up and let sperm through (such as along a sperm granuloma) and then reseal. So even if you aren't fertile right now, that doesn't mean there aren't narrow windows of time that you might have a few sperm getting through. Yes, it is unlikely that you are the father. It'd be even more unlikely that this happened if you had a sperm test and it was negative right now, but it is remotely possible. The only way to know for sure is to have a DNA test. Most likely, she is a cheater. I guess I would disclose the vasectomy to her and see how she reacts. You don't need to do it in a way like "I know you cheated because I have a vasectomy", just tell her "I can't see how this happened...I have a vasectomy. Gee, this was really unexpected." then see how she reacts. If she stays adament that the baby is yours, you got to have the DNA test. GL