Will introduction of new cat cause the existing one to spray?

Would the resident cat think the new comer is intruding on his territory and thus start spraying? Anyone had this experience? I brought home a new cat, and the old one started immediately to display territorial signs, and started to mark everything with his cheek.

Hmm....it is possible. Are they both spayed/neutered?

Most likely caused by stress. Try Feliway diffusers, Rescue Remedy, things to help calm both cats.

Did you do a "proper" introduction? Is the newbie being obviously aggressive to the resident cat?

Most important: Have you checked the sprayer for a UTI? Even if the kitty looks healthy, this is a must!

Both cats have recently been checked by vet, and they are both healthy. The newbie was screaming and hissing at the resident cat, so yeah, he wasn't particularly friendly. They are both sprayed/neutered.

Sounds like the newbie is more upset than the resident cat. He needs to familiarize himself with the new environment. I would confine him first to acclimate him to the new smells and sounds. Then begin the introductions.