He's being a dick (both times), but it's just about possible he doesn't realise it. Some people have poor social skills and/or aren't very empathetic and don't think about how their words might impact others, so it could just be that rather than deliberate malice. You shouldn't have to put up with it though, so challenging it is a good thing if you feel able to do that. Assuming he's just a co-worker rather than one of your seniors, I'd suggest speaking to him directly in the first instance, and giving him the benefit of the doubt that it's thoughtlessness rather than deliberate malice. This can be scary to do, but it's good practice. Any phrasing like u/steelspy suggested, or similar, is fine - just try to keep a neutral tone, relaxed body language, make eye contact and raise the issue calmly but directly. If you get a hostile response or if he continues to do it, or if you don't feel able to do the above, that's the time to escalate the issue. Speak privately to your line manager in the first instance rather than going directly to HR. Edit: I also remembered one of my golden rules for dealing with people: when there's any kind of an issue, it often helps to seek further information from the other person about it before you do anything else. In this instance, you could say something like "look man, I have a speech difference, it's called stuttering. I noticed you seem to comment on it pretty often and I'm not sure why, so is there any particular reason for that? Like did you have questions about it?" Obviously you would have to be *super brave* to take this route, because it involves being open and vulnerable with a person who is possibly already just mocking you. So, it's totally your judgement call whether you're open to that kind of an approach.