I had this quite a while ago (although not for prostatitis) - it can hurt quite a bit. Make sure you are properly anesthetized (lidocaine gel) and ask if you can get pain meds after, as it might hurt (a lot) when you pee.
Maybe newer (slimmer) fiber optic scopes will reduce the amount of pain compared to when I had mine done. I hope so.
Sorry - I don't mean to scare you. Good luck with the procedure and hope all turns out ok.
Also check
http://en.wikipedia.org/wiki/Cystoscopy.
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